Definitely give them some lead-in time to absorb the idea, at least a week! If you see them regularly, it is amazing fun to discuss and plan together a bit.
Once the big surprise has been revealed, you (or their parents) might want to give them a few little gifts along the way, specifically things they would need to pack. some water shoes, new bathing suits, a Disney T-shirt or pyjamas, new toothbrushes, maybe Disney-themed ones. They don’t have to be expensive items. anything wrapped is fun for a child. I still do this for my grandsons, who will be 16 and 17 on our upcoming trip! In fact, I just ordered them matching boxer briefs with pink flamingos to tuck away for one of those surprise days. They will love them...fun, fancy underwear and socks are right up their alley.
I loved the Christmas tree idea above. You could also make some MM cupcakes, and gather to watch a WDW planning video, and announce that is what you will be doing.
what an awesome Christmas gift!!!