Actually, that was not your opinion. It was fact. Child in adult area/function equals inappropriate. The whole "they weren't bothering anyone" thing (not just from you but anyone) is wrong as well. I find myself falling prey to it as well. Rules are rules, and they apply whether the child acts like an angel or is screaming their heads off. Rules are rules, and they apply to the kids who quietly blend in without notice and the ones that run around like chickens with their heads cut off.
Example: On our cruise, there were a couple of teens who got into one of the adult hot tubs. They probably COULD have been 18. But, they acted very much like young teens. I wouldn't say they were behaving badly; but they were giggly and loud, and well, teenagers. One of the CMs did come and explain to them they had to leave the area. They either didn't understand English or they pretended not to. This went on for a while of him trying to get them to leave and them being/acting oblivious. Eventually, they did leave the adult area. I felt a lot of respect for the CM but found myself rather annoyed at the young girls.
Similarly, there was a boy, pre-teen or young teens maybe. His family was sitting in the first row of loungers, and he came into the adult area. He sat down on the pool ledge, faced the loungers, and spoke to them for a while, 15 minutes maybe. No CMs ever approached them. Now, he wasn't causing any trouble, so at the time, I didn't think much of it. But, after the fact, I kind of scolded myself on my line of thinking, because they both broke the rules. One is not better than the other.
This reminds me so much of the argument about who to blame for badly behaved children- the teachers or the parents. I disagree with your assessment. While I would not say it is Disney's fault persay, I would say they do share a certain amount of responsibility. Yes, parents are at fault for not teaching their children to follow the rules, behave appropriately, and what not. BUT, Disney should enforce their own rules. If they let things go, such as MDR dress code and children in adult areas, they have themselves to blame for people not following said rules.
DCL does have every responsibility to all of its guests to enforce their policies on board.
So, instead of the back and forth of whose fault it is, I'd like to leave it at: everyone is responsible from the parents teaching their children to do the right thing to DCL enforcing their own rules.