There is nothing more annoying than someone watching and drilling you with questions and small talk while trying to setup camp. I've had the same thing happen with adults. I just tell them I'm busy right now and would be happy to introduce myself later in the day, but I can handle this myself and I need to concentrate on what I'm doing. Usually they leave.
One thing I don't want is someone's else's kids in my site during the setup or breaking camp. Too many ways for someone to get hurt or forget something.
Now in this child's situation as others indicated, he should have been under parental supervision, especially if he was a little challenged. I'm my experience "unsupervised " kids can be worse than some of the dog stories we hear constantly.