I'm going to try to get a guy and his two kids and his dog kicked out of the dog park.
That seems uncalled for. Why would you want to get them kicked out?
Their dog was acting aggressively.
I can understand your reasoning for doing such a thing. You don't have to feel badly about alerting people to other's bad behavior. If the dog is dangerous then you should have them kicked out of the park.
That's exactly right. I don't want anyone to get hurt.
You're doing the right thing. Dog parks are great, but if a dog isn't properly trained and could injure a person or another dog, then they shouldn't be there.