Main St Bakery in MK.....but just to pre-warn, this place gets really busy in the morning, so I would try to get there as soon as the park opens. I've seen it packed during value season, so I can't even imagine how busy it is during Christmas!
IMO, your best bet on Christmas morning is to just grab something from your resort CS place. Where are you staying? Most WDW resorts have great breakfast options at their CS restaurants.