What you described is the classic pit fall to dog parks. You don't have control of other peoples dogs and can only hope that they do. Unfortunately, a lot of people going to dop parks don't have control of their dogs and a lot of dogs that shouldn't go to parks are.

You need to determine your willingness to allow Charlie to play and socialize but possibly come across another dogs that may not be friendly or well socialized. The best advice I can offer is to know your dogs body language and that of the other dogs around and leash Charlie if you suspect things aren't quite right. I would rather leash the dog just incase than worry about my dog being attacked. I am personally on guard everytime I go to the dog park. I don't let my guard down for a minute and always keep an eye on my dogs when other dogs are around. It only takes a split second for a dog fight to break out.

BTW... Welcome to Pet Talk!