I'm another vote for August, due to crowds.
We also are looking at mid-August 2020 for a family trip. Sounds like my kids may be similar ages to yours. Next year we will have a daughter 2nd year of college, son 11th grade, son 7th grade. College kids CAN'T miss a week of school. Boys could, but the catch-up they'd have to do would suck. I'll admit, I didn't even consider Christmas time, although that could work schedule-wise. Too busy and more expensive.
But that's not accurate. You are not packed in, maybe on Christmas Day itself, but the rest of the time its no worse that 4th of July or any other holiday.I don't consider being packed in like a sardine to be a "wonderful time" by any stretch of the imagination.