A skatepark is an unsafe environment for most little kids, and this is a good reason why they shouldn't be allowed in parks. They don't know right from wrong at that age. And they don't know skatepark etiquette.
This is the worst. I'm not really sure what you can do. Tell the kid to stop, he doesn't. Try to find the parents, but that is probably not possible. Last resort is to move away or just sit there until he gets bored.