My first instinct was to say, "Send them with the grandparents!"
But actually, we rarely have crankiness problems with our kids. My top three suggestions are: (1) Bring a stroller for everyone not big enough to ride a big kid ride, unless you don't have enough people to push that many strollers. (2) Pack juice boxes and small bagged treats to combat heat, munchies, and boredom while waiting in line. (3) Emphasize ahead of time that "We have happy hearts at Disney!" And then bring that up every time any crankiness starts.
We did NOT go back to the resort for naps midday, but my kids were fine to nap in the stroller. We did kid stuff in the morning, had lunch, then let them nap while we cruised around places like Epcot, or did shopping.
Also, set limitations on purchases before you go. Give them a set budget and let them shop for what they really want during the week (but make sure you'll have time to go back and get whatever they decide on). Or buy some "souvenirs" before you leave -- a small stuffed toy at the Disney Outlet, trading pins off of ebay -- for "new" things to entertain them throughout the trip.
Before you leave, you can make t-shirts (you can buy shirts, transfers, fabric paints and glitter) to wear on your trip. If they look homemade, you'll get lots of CM comments on them. We also made our own autograph books, which were a HUGE hit with the "face" characters. Peter Pan and Wendy, in particular just GUSHED over them. (We used the format that was featured in a link here last August.)