I've gone to WDW at the same time as the PW kids for 13 out of the last 14 years. I've stayed at Music with then once. To be honest, it wasn't that bad. The food court and buses could get chaotic though. Now I stay where the kids don't stay!! As far as the parks go? Early in the week there is little impact. The kids are in competitions, so they're busy and their families are watching them. As the week goes by, and as more teams are knocked out, they hit the parks. So, usually Thursday through the weekend it can be a little busier in the parks.
I've been in the parks at night, and when I saw a bunch of the kids coming? I headed in the opposite direction. I actually found the kids to be better behaved than some of the adults!!!