Hi,
Congratulations on your new baby, what a cutie. I have worked with feral cats and done TNR (trap neuter release) for years. For many of them there is a window of opportunity and since this little one wants to be with you that is a great sign. Here are some pointers I can offer based on been there, done that, lol.

Your baby needs to be confined and the bathroom sounds like a great place. Take him out only when someone can be there with him and then put him back. It will be a safe place for him. He needs to know where his litter box is and getting lost in a large area when he needs to go potty is setting him up for failure. If he becomes too accustomed to doing his own thing independently it will be harder to fully domesticate him.

Handle him as much as possible. Pick him up, fiddle with his paws, ears, tail etc. The best bit of advice I can give you in addition to keeping him confined is that I really wouldn't free feed him because having him rely on you to feed him and sitting with him while he eats will increase your bond.

I know you want DD to take responsibility but please help her since this is her first time doing this sort of thing and the better it is handled from the beginning the better chances of success.

Good luck and give him/her a head butt from me. Jules