Do you want Epcot Festival of the Holidays and all that entails? If so, you have to go 28-5 as their holiday stuff only launches around Thanksgiving. My girls (10 at the time) loved the holiday storytellers, Candlelight Processional, and holiday Voices of Liberty. If this is a Disney "Christmas" trip, you have to pick that week to really get all of Disney Christmas. That said, you'd be arriving Black Friday and starting the trip during Thanksgiving weekend, so at least the first few days of that trip are going to be very busy.
If the goal is just a family visit, not specifically a visit to see Disney at Christmas, then 11-14 to 21 will probably be cheaper flights and lower crowds, since it's further from the holiday.