Not out of bounds at all. I would have done the same thing. And even knowing it might not be popular, I will also say something to someone else's child if they are being outright rude, especially to their parents/grandparents. I can't stand to hear/see a child behaving like that.

Ex: a couple months ago when I was walking into a building, a child came rushing up behind me to enter with me; his mom was not too far behind him. I held the door for him and said now you can hold the door for Mom. He said, I'm not holding the door for my Mom. I quickly responded, you better hold that door for your mom. Mom heard me but didn't really respond one way or the other.