Yes, we are going! The kids have had some better days (even with some pretty low moments) recently, and my gut feeling is that they will put on their proper "public" behavior there.
Luckily, we have done a lot of advance planning (the trip was originally scheduled for last December). We've talked about everyone's must-sees and have put them in an order based on how long the lines for each attraction are likely to be at a given time (ex: do Splash Mountain 1st), as well as their location relative to each other. The kids have pretty much memorized the preset touring plans for each park!
I printed out a lot of the great ideas people have shared here. The past few days we've talked about consequences for griping, arguing, etc. - the kids actually agree that offenders should take a time-out that requires skipping the next ride with Grandma while the others still get to go. I'm sure if it's necessary it will happen only once!
Well, we're about 60 hours away from departure. DD thanked me today for all I've done to save and plan for our vacation: she says it will be magical just to share it with her family. Now THAT'S my girl!
Thank you all for your advice and support.
Lori