I'm sure you're already aware, but it's worth noting that Nintendo produce a 3DS aimed at children called the 2DS. There are two key features of the 2DS.

1. No 3D screen. Nintendo warns about the effects of a 3D screen on a child's eyesight.

2. No hinge! Less chance of being broken by folding it back too far etc. Children tend to wear the hinges out on these things.