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.