There are misbehaving passengers all over the ship - not just in the adult area.
I'd love a cruise where everyone followed the rules all the time.
I'd bet some of the people on this thread complaining about kids in the adult areas are chair hogs, reserve seats in the theatre, board a ship while sick and lie on the health questionare, show up late for dinner, don't follow the dress code in the MDR, smuggle alcohol on the ship, don't admit to being sick when they should be confined to their cabin, have entered an eating area without using the hand sanitizing station, have used a bathroom without washing their hands and much more.
I've seen all of the above and much more. It's not a perfect world, people break rules all the time, deal with it, and try and enjoy your holiday.
That being said, this is the cruise gripes board, so I will allow you all to continue with griping. Happy Cruising to all of you.