So here i was sitting at my computer looking on derpibooru when I saw this image. https://derpibooru.org/1161470 Which in turn inspired this idea.

Celestia is hit by a spell, say maybe from Flurry Heart due to a minor magic surge when she sneezed. As a result Celestia, is De-aged, most of her memory erased (she remembers her name and her sisters name), and her cutie mark disappears. Now while the Adults are trying to find a spell to fix Celestia, she's currently hanging out and playing with the CMC, and getting covered in tree sap in the process

Could be good, except the problem is that at that time the Crusaders do less crusades as they already discovered their cutie mark.

5248317 Oh, my God, that idea is so adorable that I can't help but at least try it.
5248329 In "On My Marks" they are still the Cutie Mark Crusaders. They don't necessarily have to go on Crusades in the story. They can just play around and have fun.

5248410 Yeah, but the crusaders without crusading is like take a sandwich with only bread, nothing more. You could eat it, but it's not the same.

And I only used flurry heart as an example, there could be other ways one could de-age memory wipe and de-cutie mark Celestia Writers choice and all that

5248429 Watch "On Your Marks." While they may not be crusading to get their cutie marks, they are crusading to help others. This episode is tied in with "Crusaders of the Lost Mark", which is perhaps the best episode of the whole series, in my opinion, at least.

also depending on when the story takes place Flurry may not have been born yet and Diamond Tiara could still be a bully.