There are no guarantees, of course, just as there are no guarantees at a playground that none of the kids have something contagious. Or that the kids aren't going to get into a fight. Or that one of the kids isn't carrying a knife. We accept these risks and evaluate the situation when we take kids to playgrounds, and we'll have to do the same at a dog park. This is why we like to have adults around at both kid and dog playgrounds.