I'm thinking of bringing my son to the dog park
Do you usually bring your son with you?
Sometimes, but I might bring the dog instead
You should take your dog to the dog park if you're visiting it. They'll be happy spending the day out with other dogs and will also get plenty of exercise. If your son wants to come as well, then you should bring him.
This is too complicated. Why don't I take the kid to the dog park, he will get way more exercise there than at home.
That sounds like a good idea, if your son is interested. Obviously, it's great to take your dog to the park, but if your son also enjoys playing with the dogs, then it would be fun for all three of you.