Since my Yuu is the same as the Reader Insert I first created for my BNHA fics, I have received this type of request many times in the past since I've written fics where I've deaged others but I never did it with her, and because of her lack of a proper childhood, she's the one who could benefit from getting turned into a child more than anyone.
The reason I've never written such a fic is because I am not confident about writing from a child's POV since kids can be kinda hard for me to write especially if I'm actually getting into their heads which I would have to do cause I prefer to write from 2nd person POV.
Another problem is that meeting a child Yuu would raise a lot of worrisome questions to anyone who sees her cause her unfortunate upbringing would be pretty obvious just from interacting with her since she'd be wary of all aduts, skittish, really quiet, and would just overall not act like anyone would expect a normal kid to act.
It would honestly be a really awkward situation since Yuu wouldn't trust anyone and she'd just wanna hide away rather than deal with anyone since, to her, there's always the risk that someone will hurt her, which would obviously be very heartbreaking for a lot of people.
Since I'd rather not have Yuu have to deal with the aftermath of that, I'm very reluctant to write that kinda fic since I really don't want everyone to find out about her past.
In order to avoid that, Yuu would have to be younger than she was before she ended up in Japan's underworld so 3 years old or younger and I'm really not confident about writing from that young a POV lol
That's why the only way I could see myself writing a deaged Yuu fic is if Yuu was only physically a kid aka her mental state remains unchanged since that would make it so that she can avoid raising a lot of unwanted questions.
Or I'd make it so that her mental state is that of a child but at the same time she's still aware of her circumstances and who everyone is so she can avoid doing or saying suspicious things so like she tries to act normal but will suddenly be overcome with the urge to do something a kid would do like wanting to go play and run around.
The latter could be doable for me, so I would consider possibly writing a fic like that one day, although I can't make any promises.
Regardless of what kinda deaged state she ends up in, plenty of the TWST boys would wanna dote on her, and that would definitely be the case for all the adults, Crewel and Sam especially lol
Trey would absolutely be the #1 big bro contender cause of his experience and cause he knows she didn't have much of a childhood due to her being similar to Riddle in his eyes, so he would spoil her as much as possible haha
I agree that Yuu deserves a chance to be a kid, and I'm sure it'd be a lot of fun to write her getting spoiled, so I will keep this idea in mind ^^