Welcome to the Disney Parks Moms Panel, Kristy!
If by "soccer event," you mean Disney Soccer Showcase, how exciting! My prince and his team will be there as well, so I'm right along with you in planning around this amazing experience.
For Christmas Day, you are correct, Magic Kingdom will most likely experience phased closers throughout the day and potentially reach capacity. If you're looking for an alternative suggestion, I highly recommend enjoying your day at Epcot! With its large "foot-print" and high guest volume, it rarely closes for reaching capacity.
Experience Christmas like never before and immerse your family in the
Epcot International Festival of the Holidays. Enjoy lively storytelling of what Christmas is like around the world as you savor delicious, seasonal recipes from the Holiday Kitchens. Epcot's World Showcase Pavilions come alive for the holidays - I absolutely love to explore and learn all about different holiday traditions. Next, you can rejoice in the holiday sounds of JOYFUL! as they fill the air with gospel, jazz, and R&B Christmas music. You can also send the younger ones in your group out on a scavenger hunt of sorts with Chip & Dale's Christmas Tree Spree, a fun way to get to know the ins and outs of World Showcase.
I hope you like my suggestion, Kristy! Please do not hesitate to reach out to me for any additional advice regarding your free-time, or your soccer time! When you compete at the ESPN Wide World of Sports Complex, you're in the middle of the magic.
Wishing you a Merry Christmas and lots of soccer goals ;)
Faith