My two cents!
Speaking as someone whose mother pulled this on her, don't tell them. Just say your going away for some adult only time. (Which is much deserved I'm sure!) Think of it this way, if your favorite vacation destination was a tropical island or a ski chalet, would you feel as guilty going without them? It's only the disney guilt that's bothering you. Just remember you get some mommy only time too!
Maybe you could tell them your going to florida, so if you want to bring things home you can say you made a quick visit to Mickey just to buy some things for them.
Don't feel guilty, you deserve it, and your children will have many more happen
disney vacations to come, I'm sure even if they do find out where you went in the future they won't begrudge you. ( I still don't for my mom) But if they specifically knew you were leaving them to go to WDW, they'll know what they're missing and then might feel hurt.
Unless you think they are mature enough to understand the concept of "mommy needs a break for a bit, and loves WDW and really really wants to go, but can't take you this time." I would stick to what they don't know won't hurt them policy.!
Hope you feel better about your decision and HAVE A GREAT TIME!