We surprised our kids last time but gave them 10 days notice which I think was good. I wanted help packing! Plus they would be missing a little school and I wanted them to be prepared. The teachers knew about it before the kids though.
At 10 days out, we prepared a treasure hunt. Each hidden envelope contained two slips with math problems, one for each girl. Each answer stood for a letter of the alphabet. When they collected them all, they solved the problems and filled out a code that read "Look at the November calendar". When they lifted the calendar page, they saw "Disney World" in bright colors on our dates. Screaming and falling on the floor ensued! For your younger ones you wouldn't need to incorporate the math, but could have any sort of treasure hunt.
Another idea I considered was mailing them letters in Disney font from their favorite characters.
Personally I would give them a little warning, even if it's just the day before. But you're the one who knows best how they might react!