Are the parks open Christmas Day? I really want to surprise the kids.

Yes they are open Christmas day.

Here is the park schedule for that day. https://disneyworld.disney.go.com/calendars/day/2020-12-25/

This year, part of it will be: Does Disney World really close? https://www.dadsguidetowdw.com/does-disney-world-really-close.html#:~:text=DISNEY%20WORLD%20doesn't%20close,incoming%20guests%20on%20Christmas%20Day.&text=It%20is%20in%20DISNEY%20WORLD,at%20the%20Magic%20Kingdom%20story).

Honestly not sure, but in any case if you are planning on visiting you need to purchase tickets in advance and call/use the app to reserve the day you will be visiting