Always relax the bow hairs between practice, or they will wear out, and in the worst case, the bow will break.
So before playing, tighten the hair by turning the screw until the gap at the narrowest point is approximately as wide as the bow stick is thick at that point. And after playing, loosen the screw again so the hair touches the bow.
Also, do not touch the hair. Rosin should be used in moderation. It’s best to do this at the beginning of class until your child gets a feel for how much is good.