Yikes. Kids should not be running around at a dog park. Period. The situation you describe is dangerous, and shouldn't continue. Your responsibility is very clear: you are responsible for making sure Megatron doesn't hurt a human being, even accidentally. It would be nice if all parents controlled their kids and if all humans knew how to interact with a dogs, but that isn't going to happen.

Mouthing and jumping is okay between dogs; between dogs and people it isn't. Dogs can easily learn this distinction.

You should immediately teach Megatron that mouthing people is not acceptable; ditto for jumping on people. I use clicker training; it's easy, fun and backed by sound, up-to-date science.

To change these behaviors, you've got to address them when they occur. Scolding him after you've called him away would do no good anyway -- the dog won't know what you are talking about.

To gain a greater understanding of dog-human interaction, I'd suggesting reading books by Jean Donaldson (Culture Clash is a classic; it changed my life) and The Other End of the Leash by Dr. Patricia McConnell are good starting points.