If you need a diaper bag, pack as you usually do. Bring a few extra supplies to stash in the car.
Shell out the extra bucks for a parking spot in the lot closest to the park. With luck, this may put you right next to the turnstiles, and it is as easy to head out to the car for a meal as it is to go to a vendor in the park. It pays off to keep extra supplies in the car for emergencies. If the park doesn't sell guaranteed parking spaces in a VIP lot, show up at least an hour before the park opens to get the best space in the lot. Bring extra things for the kids to do while you wait. You will be glad you got a close space when everyone is tired at the end of the day, or when you need to dash out to the car for a quick change of clothing or to load up on snacks.
Use one belt pack as a first aid kit. You probably won't need these items, but if someone gets stung by a hornet or develops a splitting headache, you'll be glad you have the kit.
If you are going to divide into groups, make sure that one person in every group has a tube of sunblock.
Seal cell phones in waterproof bags before stowing in belly packs. They could get wet on water rides.
Make sure that everyone has snacks. Pack healthy snacks: whole grain crackers, beef jerky and cheese sticks.
If you will be parking close enough to the entrance of the park to go back to the car for food, bring a cooler filled with ice and pack plenty of food. You'll get hungrier than you think. You can leave the changes of clothes, spare shoes and socks, and sweatshirts in the car for a quick change when it gets chilly. And it will, if the park is not in the subtropics, and if you stay after dark.
Use a carabiner to attach a plastic cup with a handle to an adult's belt loop. Water bottles are nice, but are often prohibited on rides because they are hard to secure. A plastic cup with a handle can be attached to the belt loop and filled at the nearest water fountain -- which beats lifting preschoolers to fountains for long drinks.
