It's definitely very crowded. Will you be staying on-site?
My family always does Magic Kingdom on Christmas Day. We stay on-site at the Port Orleans Riverside. There will be EMH from 7am to 8am and the park will open to the public at 8am. We always say that we'll get to MK for the full morning EMH, but we never do. After all, we take an early flight on Christmas Eve, so while we try and be ambitious about getting to the parks at 7am, we usually leave the resort around 7:30am and get to the park for 7:45am.
By 8am, you definitely notice a crowd, but it's far from overwhelming. We'll usually head to Space Mountain first. It'll be around a 20-minute wait. By the time we finish on SM, the park is open to the public. Like I said - nothing crazy, but you can see the wave of people entering the park. I remember our first time at MK we were surprised at how "empty" the park was in the morning. I say empty because it's comparatively speaking to what's expected and what we've seen. Sure, the ride wait times went up quickly. Space Mountain was around a 40-minute wait by ~9am, but it wasn't CRAZY. The crowds didn't really file in until 10am. By 11am, there were phased closures and by 12pm, the crowds were a bit overwhelming (hard to move around by Small World, the hub was flooded with people, Main Street was hard to navigate, etc.) The ride wait times were astronomical. If I remember correctly, 7DMT was around 3 hours long, Space was 2 hours, Thunder Mountain was 90 minutes. The cult favorites, such as Peter Pan and Winnie were around 60 minutes. The Haunted Mansion, for some reason, was 15 minutes.
So I'd say this - Don't shy away from Magic Kingdom because of the crowds. We arrive early and get in as many of the rides as possible before calling it a day and heading back to the resort. Get your FP+s for rides like Space Mountain, Thunder Mountain and maybe a cult favorite. Don't even bother trying for 7DMT - you won't get it. Arrive for EMH if you're on-site or rope drop if you're off-site. Book your FP+ for 10am, 11am and 12pm. From 8am to 10am, get in all the rides you don't have FP+ for. I'd recommend one of the mountains (if you don't have a FP+,) the Jungle Cruise (this will hit ~60 minutes by 9:30am,) Buzz Lightyear (another ride that will skyrocket shortly after opening,) the Haunted Mansion and Pirates. Around 12pm, you can grab lunch and/or head to Animal Kingdom. AK is the least crowded park on Christmas and many sites will actually recommend it.
OR, try this:
Do MK in the morning. Get in as many rides as possible and actually book your FP+ for Animal Kingdom (Dinosaur, Expedition Everest and the Safari) in the afternoon. Get in all the rides you can - Space Mountain and/or Thunder Mountain at rope drop, Jungle Cruise, Buzz Lightyear, Small World, a few cult favorites... around 1pm, head to Animal Kingdom for afternoon FP+ at AK.
If you want some tips and tricks for Christmas at Disney, check out my site at survivedisxmas.com
