Celestia tends to be cranky when she's stressed. So when Discord wakes her up to complain about feeling like Fluttershy has been ignoring him. She doesn't take it well. After a heated argument, a magical mishap between the two causes them to regress in age.

You are Anon, a member of Luna's royal guard and a unicorn with a special talent for transference magic. And when a magical mishap happens in Celestia's quarters, you become charged with being little Tia's new caregiver for when Luna is busy with her royal duties. And you soon learn that Tia was not an easy filly to take care of.

And between Tia, Luna having no experience with raising children, and Fluttershy's issues with little Dissy, life is certainly not going to be the same.

Inspired by Moonie Shorts by Eighth. Gene's point of view can be found in Little Dissy.