For prevention, it doesn't hurt to try speech therapy. Preferable ones like Hollins Stuttering Institute that have something similar to speech sound stimulability. As for growing out it... I think that depends on how the child's attitude about their own stuttering and how much it affects his or her confidence when speaking. Also it depends on how life experiences they've had such as bullying or other traumatic events. As you get older and you develop confidence in other things and gain competence in a profession or hobby, I find you stutter less and less.