Teaching Ein that kids aren't going to hurt him?

Ein loved kids... that is until he got trampled by an unruley 2 year olds who's parents didn't understand the concept of ask before touching. Since then, he's petrified of any kid that are standing up. He pretty much freezes solid and won't move, but will growl and stand off. He loves babies though, he loves cuddling my niece, who is 7 months. I've had NO luck teaching him that kids aren't going to hurt him. Now I'm asking for your input.