Son of the Sun

...Princess Celestia? Are you sure?

Yes, thoughtful student. I am certain. You will be as well, one day.

Thank you for saying that. Thank you, princess.

Nicholas was told by Celestia to start walking, and he did as he was told, ears dipped and sniffling.

She had him trot in front of her, telling him to quicken his pace when he was beginning to get in her way.

Soon though, Princess Celestia sighed in irritation, yanked him up by the nape of his neck, then hurriedly dashed into her throne room as he yelped.

Princess Luna was in the throne room, casually sleeping in Celestia's throne of all places.

Celestia brought Nick before Luna, deciding to deal with two annoyances at once: his crying and her sister's snoring.

She promptly spit him out directly onto Luna's chest, making the mare lose air and nap-time when the colt impacted.

Luna groaned in pain, cursing out loud until she opened her eyes to view a frightened child sitting on her stomach.

She raised an eyebrow, wondering, "Sister, where did you-"

Celestia growled at the current tread on her territory, "Get. Off. My. Bucking. Throne."

Luna rolled her eyes, picked up the child in her front hooves, then flew to the side of the throne.

Snorting in insult, Princess Celestia took her rightful seat of power, demanding her sister got on with whatever she was babbling about, "Hurry up with your shock and awe, freeloader. I've got a nation to rule, unlike you."

Luna scoffed, holding the child in her hooves as if he were a foal, "Awww, how cute. Did you paint this one up? There's no way this is him."

"What did I tell you?"

"You said to dress up like a loser for awhile. Like cosplay, only lame."

"Aside that!"

"That this one wouldn't backfire?"

"Stop with the damn flame puns already. Before you get actually roasted, you unsociable nerd."

"I will as soon as you stop being so rude to me."

"Apparently, you wish to get fired."

"Okay, now which one of us is annoying who with the damn puns?"

Nick exclaimed as his view was spinning, "I hate heights! Please, please, please put me down, Princess Luna!"

Princess Luna immediately dropped to the floor, holding him away at arm's length, acknowledging, "Sounds like him. Definitely is acting strange."

Princess Celestia was ashamed at what was currently being said, face in hooves yet somehow screaming without issue, "You absolute moron! I said that this is my son, so he is my son! Yes, admittedly, it will take time to actually make him grow accustomed to being with me after having to put up with that weak-minded, loving mare he spent most of his life with!"

Nick did point out, "My mom-"

Princess Celestia turned his head sideways magically, showing her teeth as she hissed, "The next few words that exit that muzzle I let you still have had better by about me."

Nick, still being held by a now-worried Luna, gulped before explaining himself, "My...momalwayssaidIcouldthinkofyouasasecondonetome!"

"I hope you have a really good grip on my sweet, disobedient child, sister," Celestia declared before winding her hoof back.

Whack!

Luna was really glad for placing him a hoof's space away from her main body because of two reasons. The first being pulling him in closer for a tighter grip would've meant she likely get a taste of his medicine. And, second, he...pissed himself from the backhoof.

He shivered so hard his teeth chattered. He could feel a warm, fresh red mark on his already deeply red cheeks as an even warmer liquid coursed down his fur to drip onto the throne room tile.

Princess Luna asked of her sister in surprise, "Why'd you do that?!"

Princess Celestia shrugged, shaking her head at the state of her colt, "What? The slap? Oh, calm down. Every child needs, at least, one good smack to the face every once in awhile for them to know their place. It's like...Childcare 101. And, now you know why I'm fond of the color red as well as purple. Such amazing colors to smack around because of how rarely a mark or bruise is left...seen. Barely looks like he just got discplined, huh?"

"I meant in the throne room! Smack him around all you want. Not my damn kid."

"Oh. Yeah. Here, I can fix that."

Celestia proceeded to cast a second Age spell on Nicholas, turning him into an actual foal of around two and a half years, saying in joy, "There. We're all good now."

After calling for a maid, Luna explained to her sister the concept of cause and effect, "You slap a child as hard as you did, they're probably going to pee wherever. The problem I've been reacting to is the piss that was collecting on the floor! I've seen rocks less dense than you!"

Celestia responded to the maid's appearance, "Can you please diaper my son? He's a tad...unruly, so he'll be spending the rest of today in that age. If he behaves...how I want, he can be aged up a year a day."

Just as the maid sighed once reaching for the child who still had his mind for awhile, Celestia retracted her request, pointing to Luna to advise her, "Wait. No. You diaper him."

Luna would've shoved the still-scared child into the maid's hooves if Celestia's magic had let her, which it didn't.

She returned to wobbly holding the child at hoof's length, adding on with a whine, "Why me?! I'm no damn nanny!"

"Because...I'm bigger, stronger, faster, more powerful, and twice as beautiful as you? And, by extension, willing -- and able -- to beat you into moondust?"

"...Whilst most of what you said is debatable, one of those points isn't. Bitch."

Celestia would've attacked her own blood relative, but Luna teleported a few hooves away.

As she poofed everything she'd need for supplies from thin air, she still had to shout out questionnaires towards her sibling, "Is he really so different than before? I thought that-"

Princess Celestia twirled her hoof around, remarking, "Events can be changed, Luna. What happens one time doesn't correlate with what happens at some other time. Don't ask me how it works because I only understand what I need to know to get by."

As Luna finished wiping the foal down, who still was crying silently, she threw in, "Well, it must've been pretty damn different. I don't remember ever having to change his diapers, yet here I am. Playing nanny!"

"You pull it off so well."

"Maybe if you'd do stuff like this more often you'd still be like me."

"Buck off. I'll fix everything. Just takes time."

"Can you afford that time, sister?"

"Of course. And, I'm certain the same rules apply to my little baby right there as well. Has to."

Just as Luna finished laying the tabs down onto the foal, Celestia ported him over into her lap, grinning at the foal who now was curled into a ball for protection, "I'm sure you will. After all, she never developed your potential to the fullest, did she?! No, she didn't! Nope! But, I will. She'll never get a chance to use you. For your all mine now. I'll give you all the guidance you deserved as a child. But, you won't have to die like your brother did. Oh, no. That was such a sad mistake. One I plan on never happening again!"