We decided to take a trip without the kids while we were at WDW in June, and we told them right there (ages 12, 9, and 4). They didn't bat an eye at the time!
Of course, now that our trip is getting closer I expect a little complaining, can't really blame them, but they'll have a great time with grandma and papa, and we're building that up to them. It'll be a special trip for them, too, only it won't be to WDW this time!