My son 3 and a half says his wrist is sore inside. he can still move his fingers and wrist up and down. fell at school late this afternoon. a sprain?

Torus Fracture: At his age, children don't sprain, they break. The good news is that, unless the wrist is noticeably deformed, it will heal uneventfully. All it needs is protection with a Fracture Brace for about 3-4 weeks.

See details: It is likely a sprain or contusion. A fracture is less likely as movement does not cause significant pain. If the symptoms persist or pain increases have the wrist evaluated.