I've been to WDW several times at the same time as the PopWarner kids...one time at Music where they were staying. I have to say that I didn't have any of those issues that others speak about. Yes, the kids could get rambunctious, but we're talking about teens/preteens here. They're loud everywhere!! The only impact I had was if I got to the food court at the same time they did...then it got a bit zooey!! So, I got there before them..I like to eat earlier rather than later so that worked quite well for me.
As for the parks, the kids weren't really in the parks during the day. At night they would be very evident..lots of 'kid herds', with the shouting, running that would come with a group of 8 teen girls!!! But it just wasn't that big an issue...at least not for me.
I think they use POP mostly for overflow from the AllStars. I was at POP a few years ago, for a few nights at the end of the PopW run, and I didn't notice that many more kids than usual.