How to help my dog to accept small children?

My dog a pitull rescue, who I got a year ago, (I know nothing about his past. ) He is five. when I take him on walks, he is great, he smells people, lets people pet him, but when a little kid 3-6 runs by him he can't help but chase the kid. I don't know why he gets like that, It really limits the places i can go with him, which really sucks,and he has this problem with black men that look like gangsters, I dont know why. He has gone to the park, but tends to sit down and stare at little kids playing ball like he wants to chase them! My bichon is the same way with little kids. if they run by her she will grab their jeans, or shirts and pull them down! How do I let my dogs know that little kids are okay, and to leave them alone?