I guess the socialising process works both ways, even in a dog park!
Dogs, muzzled or not, probably should be encouraged (if possible) not to rough-house with children much the same as we'd encourage our kids not to poke or hit dogs with a stick.
For whatever reason, my youngest dog despises children and will growl and bark at any child that approaches it. Because of that I make sure the dog is under control, verbally or physically, especially when I see smaller children about. It's not the child's fault that my dog is snappy, it's mine.
My other Miniature Pinscher on the other hand is gentle and I am happy to let her off-leash with children about ... Getting her back on-leash is another story though!