I've had a boxie for two years. I also have numerous geckos, a snake, a red footed tortoise, dog, cats, cockatiels, fish...
Of all these, I've spent the most in vet bills on the box turtle. Definitely not an animal for a child.
The list of things that can go wrong is longer with the boxies. And, they are not as personable as other turtles, which can make a child feel frustrated.
Do you want a puppy in a shell? Go for a red footed tortoise. The one I have has been sturdier, not prone to infections as the boxie, does not hybernate, and eats like a little pig. Many say that of all tortoises, this is perhaps the species with the most personality.
Still, plan on taking your turtle to the vet for an initial assesment, a fecal test, and maybe blood, if the vet feels there is a reason for it. That can be an expensive matter all by itself, as you should take the animal to a reptilian specialist. In some states, tehre's only a handful, and you may have to travel to get to one. A vet who focuses his practice on dogs, and cats may be spinning his wheels with a turtle. In the end, you'll lose more money.
So, I'd read a little more about other turtles, and tortoises. If your child is anything like mine, guess who will end up responsible for the care and husbandry. Select what you see yourself as being capable of doing, and check your nearest turtle rescue facility. It can be a very good, hands-on start and you may have all the contacts you need right there.
Wanda