We were at WDW the entire week Pop Warner was there in Dec. 03. I really didn't see too many of them at the parks, but they did crowd the food court at POR.
I think most of them are busy at the various competitions during the week, so crowd size at the parks isn't affected.
DO NOT let the fact that Pop Warner will be there change your plans. We had a great time and the holidays are extra special at WDW. No waiting in line, comfortable weather, and great decorations.
If you have little ones be sure to get tickets for Mickeys Christmas Party (I think thats what they call it) at the MK.
Have fun!! We'll be there too!!