Take your own drinks and snacks to the park, at a minimum. Pack yourself some juice boxes, granola bars, those individual packs of cookies/goldfish. Nothing makes a kid more cranky than being hungry or thirsty and having to wait or waiting in line with nothing to do. A little bag of goldfish crackers will make the lines go by so much faster and keep little tummies from getting empty.
Also, get yourself a double stroller because tired little legs wil make for cranky customers.
When my kids were little, I also bought little packs of Disney stickers and little Disney items from Dollarama and took them in my backpack to dole out when the kids started to get a little off their game. It helped! It also kept them from getting the gimmies. And speaking fo the gimmies and meltdowns, do NOT take them into the gift shops. Just don't do it. Have another adult in your party take them to something and then you go in the shops. When you surprise your little one with a special Disney thing you've bought, they'll be thrilled but if they actually see all there is to offer, they're going to want things that aren't reasonably priced, you're not ging to want to pay it and then, meltdown. LOL I did this with dd our first trip (she'd just turned 6) and it worked like a charm: I went into Tinkerbell's Treasures and picked her out a seet but practical sundress. She loved it and got alot of wear out of it after our trip. I have no doubt, if I'd taken her in, she'd have wanted the Deluxe Sleeping Beauty dress or the deluxe tiara at a minimum.
Oh, another thing we did (we had a rental car) was take lunches and snakcs ina cooler bag but buy the kids one treat/park/day. You can get a Mickey waffle pretty reasonably from counter service (Sleepy Hollow). The kids can split it and feel like they're getting a real treat. Some good spots for this were: Norwegian bakery at Epcot, Mrs. Potts' Cupboard for sundaes at MK, Main Street bakery at MK, Main Street Confectionary at MK.
Go first thing in the a.m. to avoid the crowds and take them back to your hotel for 'down time'. Every hotel has the Disney Channel. Even if your kids don't sleep, the down time will do a world of good.