I'd like to quickly point out that film rating are not laws. They're rules in which any theater can choose to abide by.
As far as annoying kids in a theater? I have a split opinion on that. If it's release day, first showing. Fine. Any other time. No. The reason for that is, normally, release day is awesome. There's nothing like going into a packed theater with a ton of crazy fanpeople. (Trust me I know, I was at the midnight release of Star Wars.) So it's okay then.
But when the theater isn't full and you got your three-month-old crying and that kid who for some reason has to pee every five seconds... It's bad...