Please don't panic! We went first week of Dec last year and had such a good time that we're leaving again this Fri (12/1) for a week. Here's the deal:
yes, the Disney hotels themselves fill up -- but the "outside" hotels are EMPTY. This means that, yeah, your hotel will be full but the crowds overall ARE low because you don't have the people in all the area resorts.
yes, Pop Warner -- the kids and chaperones are there to compete at events in the sports arena. Yes, they are sometimes noticeable in the parks (kids wearing their school jerseys, cheerleaders cheering, etc.) but they don't have a big impact on the crowd level. There is one night that MGM closes early because it's open only to the Pop Warner crowd -- check out the hours/calendar if that's a concern. Mousefest is nice but doesn't even move the needle on attendance.
Final note -- if you can avoid MK on 12/2, then do so, but do not panic if you can't (because you've already planned to be there). They film the ABC parade there that day. Main Street is impassable (from early morning to mid-afternoon) but you access the park by walking straight through all the shops on the left=hand side of the street and then the park is very manageable. Again, we were there on that Sat last year and although we missed the "first magic walk down Main St.", we made up for it later in the trip and very much enjoyed our day in MK. Fantasyland was busy when the parade crowd broke (between 2 and 3 pm) but otherwise no complaints.
Feeling better?