Twilight Sparkle said sternly, "I need to make sure you two both understand. Dinky Doo, this could cost you at least a decade of your life. Maybe a little more."
The pale purplish unicorn filly said stubbornly, "It's worth it. I won't let anypony take my mommy away. And you told me that someday, you can probably reverse it."
Twilight said, "There's no guarantee. If in the next ten years something happens to me, or if I can't find you, or if you can't find me...there might be no way to ever undo this."
The unicorn stomped her little hoof. "It's worth it."
Rainbow Dash ran into the room. "Foal Protective Services is getting very confused. Seems a lot of ponies are giving them bad directions."
"Who'd have thought?" Twilight said. "Too bad the one pony who knows where every address in town is, doesn't want to talk to them."
Derpy Hooves nuzzled her little unicorn filly. "Dinky, I just want you to know, you don't have to do this."
Dinky Doo insisted, "We HAVE to do this!" She turned to Twilight. "Do the forbidden spell now! Before it's too late!"
Twilight rechecked the parchments on which she had carefully scribed notes.
Fluttershy walked in. "I think the FPS ponies are getting impatient. They're putting up these posters and giving them to everypony." She put a poster on a tabletop. "It says Derpy has dropped heavy objects on ponies at least several times, seriously injuring them, and it says she's a public danger and can't be trusted to protect equine life from deadly harm. It has a big photo of what Town Hall looked like after the accident with the cloud and the lightning, and in big print it says, what if a defenseless filly had been inside?"
Dinky Doo started shaking and crying. "What are they going to do to my mommy?"
"There, there," Twilight tried to reassure Dinky, gently touching the filly's shoulder with a forehoof. "We won't let them do anything to your mommy. Now I just need you to stand under your mother's wing, facing me, and get your horn ready to channel like I taught you."
Dinky Doo nestled under Derpy's wing, and stared at Twilight with a serious look.
Twilight mumbled to herself, "Harmony, please give us luck, because Derpy and Dinky sure need it." Her horn lit up with a glowing, air-blurring cloud of magical energy. "Ready, Dinky?" she asked.
"Ready!" Dinky shouted. Dinky projected a small beam of magic at Twilight. Twilight's own glowing cloud shot along Dinky's beam, to hide the tiny unicorn and her mother within a spinning, wobbling, colorful nebula of magic.
The door burst open, and in rushed several Equestrian Guardponies along with a short mare holding a briefcase. The guardpony's sergeant held up a copy of the poster, and said, "Do you know where this pegasus is? We believe she is committing criminal foal endangerment!"
Twilight said mildly, "I am performing a very delicate magical working. I must ask you to leave us alone until it is finished."
One of the guardponies looked back and forth between the poster and Twilight. "Mare'm," he said, "are you Twilight Sparkle, the pony who was seriously injured several times and almost killed by the suspect's recklessness?"
Twilight said, "Fortunately, I was under the influence of a Pinkie Pie cartoon physics field. So I like to think I was never in real danger."
The guardpony looked doubtful. "I don't know what kind of alibi or excuse the criminal might have given you. But if you had been killed, that technobabble wouldn't make anypony feel any better."
Twilight sighed. "If you only knew how many times in the last few years I've been almost killed, and by worse than a falling anvil."
The guardpony became extra attentive. "Worse than a falling anvil? How many times has the suspect tried to kill you, and by what means?"
"I'm afraid there's been a misunderstanding," Twilight said. "I've dealt with many dangers and deadly foes. But I'm not talking about Derpy. Ever since Celestia expelled me from her Everton Special Tutoring Program, I've been out here in the real world, trying to protect Equestria from more monsters and overpowered villains than you can shake a stick at." Out of the corner of her eye, Twilight sneaked a glance at the cloud of magic swirling around Derpy and Dinky. It was still opaque enough to hide the two ponies.
The sergeant eyed Twilight with new suspicion. "Miss...Twilight Sparkle?"
Twilight said nothing.
"Miss Sparkle, it seems...possibly relevant that you were expelled from Everton. Expelled by Princess Celestia herself, you say?"
Twilight stayed silent.
"I don't know what kind of misbehavior got you expelled from that prestigious program by our top alicorn princess. I imagine it must have been quite serious."
Botching the return and recovery of Princess Celestia's long-lost sister might be called serious, Twilight thought. Although to be fair, Twilight was, please forgive the wording, working in the dark, with incomplete and misleading information. She'd had no idea that she wasn't supposed to send Luna back to the moon for another thousand years. Nor had she known just how upset, traumatized, and angry Celestia would be.
Ever since that dark and tragic night, Twilight had been forced to live with the understanding that authority's guidance can be tragically incomplete, if not all too often simply wrong.
Twilight wanted to glance at the swirling magical working again, but she dared not. She didn't want to direct even a tiny bit of extra attention towards the fugitives.
"What is this magical working, anyway?" one of the guardponies asked. "Are you sure it's really on the up-and-up? Or is it more of the kind of behavior that got you expelled from Everton?"
Beside her, Twilight heard a sizzling of the magical cloud dissipating. She prayed to Harmony that her desperate measure had worked.
The sergeant stared at the place where the swirling magic had been. "What's all this, then?" he asked. "Check these ponies at once!"
Twilight's eyes followed the sergeant's suspicious gaze. Beside Twilight stood a light purplish unicorn mare with blond mane and tail. The unicorn's flank bore an archer's bow as a cutie mark.
Pressing closely against the unicorn was a gray pegasus filly whose mane and tail almost matched the unicorn's. The little pegasus was of a usual blank flank age, and she had no cutie mark. Twilight silently said to herself, Thank Harmony! The spell seems to have worked!
The closest guardpony prodded the unicorn mare. "Who are you?" he demanded.
The unicorn cleared her throat. "My name is Far Flying," she said. "And this is my daughter Tiny Bubbles. We came to ask Twilight to heal our family."
Twilight ventured the question, "Are you feeling better, dears?"
Tiny Bubbles said in a small, foalish voice, "I'm feeling much better. I'm so happy that nopony has to lose their mommy. I just feel so full of love and happiness."
Twilight grinned.
The guardponies examined the room suspiciously, but found nothing. Soon they departed.
When the official ponies were all gone, Twilight remarked, "Far Flying?"
The light-colored unicorn said cheerfully. "I had to call myself something. I hope the name isn't too obvious, considering that my real mother and I are fugitives. But at least I seem to have gained my cutie mark by succeeding in co-casting the spell with you."
Twilight mused, "You talk like somepony older than Dinky Doo. You use different words. And different thoughts, I think."
The unicorn shrugged. "It's strange. I feel older. I have knowledge and skills that I didn't have before. But some specifics are missing. I know things that a pony learns while growing up, but I don't have memories of exactly where and how I learned many of them."
Twilight laughed. "Nopony tell the Cutie Mark Crusaders. I hate to think what kind of trouble they might get into, if they hear that this kind of spell exists."
Far Flying replied, "We'd better tell absolutely NOPONY about any of this. This plan relies on secrecy. No way am I letting anypony put my mother into a dungeon cell while she's a little foal."
"I'd better practice calling you mommy," Tiny Bubbles said. "And you should practice calling me your daughter."
"Okay, honey." Far Flying nuzzled her little pegasus filly.
"On one hoof," Twilight said, "Maybe it's better if I don't know too much about where you're going and what you're going to do. But on the second hoof, when it's time to reverse the age-trading spell, we want to make sure we can find each other to do it."
Far Flying nodded. "That would be prudent," she said. "But on the third hoof, I seem to have some magical ability of my own. Ten years from now, maybe I might be able to reverse the spell without you, or with some other pony's help."
"On the fourth," Twilight said, "It's wise to be prepared with multiple plans, so if one doesn't work out, we can try another."
Rainbow Dash flapped her wings, rising into the air. "Ten years from now," she said, "most ponies won't be thinking about Derpy's escape anymore. So when that time comes, I can do a sonic rainboom really big and high up. If I do it right, everypony within a hundred miles should see it. Even ponies who didn't see it should be talking about it. So ten years from now, all three of you can come looking for me. That's one way you can meet up without even knowing now where you'll meet."
Twilight thought silently.
Far Flying said, "And if for some reason Rainbow Dash doesn't survive, and her sonic rainboom doesn't happen? We can meet ten years and three months from now, at the gravesite. And if that doesn't work out for some reason, eleven years from now exactly. After that, I guess we improvise, or once a year anniversary if improvising fails."
"Wow," Fluttershy said. "Far Flying, you really are older now. Already thinking so seriously about mortality and responsibility."
Rainbow said, "FF is right, though. None of us here exactly live safe lives. Any of us could die protecting Equestria. I'm not saying that we will, but it's a good idea to be prepared."
The assembled ponies carefully checked their understanding of and agreement with the plan. Having agreed, they left singly or in pairs, hoping to meet up again one day.
Author's Note
Please follow me, upvote my story, comment, and join my secret fimfiction.net group! (It's so secret even I'm not sure where it is.)
As usual, I'm interested in comments about something you liked, something you didn't like, or even some of both!
Today, I used the laziest possible option for story art: https://mlp.fandom.com/wiki/Derpy. So you should give me positive reinforcement for using the time I saved to try to write a good story!
Where did this story's idea come from? I read the ~~tearjerker~~ story or sadfic Derpy's Rainy Day, and I decided to write something almost completely different. 
SPOILER: The sadfic has a sequel in which some happy ending things happen instead of Derpy dying of a broken heart in a mental institution.
