I vote for reveal a few days ahead. I did ours at 10 days out since they had to miss some school. They said that was perfect as they still had a little time to plan, anticipate, and pack the things they really wanted along, but the wait didn't drag on forever either. We did a treasure hunt where they gathered clues for a code message that said "Look at the November calendar". They lifted the October calendar page and saw Disney World written across our dates.
You know your kids best, but after reading/seeing some surprises, I would consider the following:
Don't tell them you are going somewhere else fun and then switch it to WDW. They may still want to do that other thing and actually be disappointed.
Don't get them up at dark-thirty and expect them to be overjoyed right away.
Make sure they are not missing any events they consider important without warning.
If you're packing for them, make sure you get all loveys and everything else important!
If your kids don't react well to sudden changes in plans, tell them at least a little in advance.
Don't surprise first timers if they don't have any idea what WDW is like (not applicable in the OP's case).
If your kids will be missing school and might freak out about missing it without warning, tell them in advance.
Don't do it at bedtime the night before if you want them to get to sleep