I would encourage you to assess what features the child needs that he would not get in home but get in Ultimate. The main Ultimate features missing in Home premium are XP mode, ability to join a domain, backup to a network location, Bitlocker and ability to switch to a language of choice.
Compare Windows 7 Editions
Now, the other part of your post. You want to downgrade the ultimate to home premium without losing any of the installed stuff AND you want to reformat C: . These are 2 contradictory things- theres indeed a way to in-place downgrade that would keep ones installed stuff intact. But if you format C: obviously you'll be losing everything and will need to install windows and all your apps and drivers afresh. So please make up your mind.
Heres how to downgrade in-place.
UNAWAVE - Downgrade Ultimate to Home Premium or Professional
Before starting the downgrade, best to save an image of the current install with Acronis or macrium.