I'm going to record a 6 year old to prove to his parents that he is rude.
Have you tried talking to his parents about his manners?
Yes, but they don't believe what I tell them. So, I'm gonna show them proof.
That's a difficult situation. I understand wanting to prove to the parents that you are right, but at the same time recording a child is a bit creepy and others might think you are doing something inappropriate. Do you know anyone else who could talk to his parents to verify what you are saying?
I do, but I think the video would be better proof.
Before resorting to recording the child, I would suggest having the other person you know talk to the parents. If they hear it from multiple people that their child is rude, they are more likely to believe it and take action. If that doesn't work, then you can try to work on a different solution.