Wow, sounds like an AWESOME trip!!!
I would say, make sure you have hats and sunglasses for all (the sun can give everyone a headache while looking around in the parks)
At least one good pair of comfy shoes, plenty of snack and drink items for all (water bottles, juice boxes, individual snacks, etc...but you can also buy it there if you want)
I bring some notebooks for the kids to scribble on if they get bored waiting in lines or in restaurants, planes, etc. I buy them a couple of new ones for the trips for something new.
cameras, and I like to get each of my kids a disposable camera. They LOVE it and the pictures are pretty interesting!
And don't forget to pack everyone's sense of humor and plenty of smiles!