Well, if the child is not deficient, daily requirement can be fufilled with diet containing milk, egg yolk, cheese, yogurt, mushrooms.
Before starting supplements on a regular badis please get
Vitamin D levels checked, if by chance levels are good we dont want to unnecessarily give supplements.
Still of you want to go ahead depaite confirmed deficiency start with lower dose of 600-800IU daily. Can go upto 1000 IU for 3 months. Get tested done thereafter.