I think you ought to keep him and lease him out to someone different than the little girl. Those "good as gold" kid's ponies don't come along too often, and they're SO valuable. Plus, kids' ponies get outgrown all the time. The nice beginner level pony that is perfect today is not going to be the horse that beginner needs when they move up. I can't say I trust parents who want a little kid who will inevitably outgrow the pony to have her "own" pony just for the sake of it.
If you can't lease, can you let your BO use him for lessons and have her knock some money off your board each month?