Wow, you sound almost like our family - This October will be our first trip for our children - DD 4 and DS 2 and the third time for my husband and me. (Although I have been about 15 times before that with my parents.)
I plan to use NetGrocer and pre-order snacks (and maybe diapers,wipes, paper plates, etc., too, so I don't have to lug them accross the country- we'll be flying) to be sent to the hotel so I don't have to spend precious vacation time grocery shopping when we arrive. Our kids like to nibble through-out the day and eat first thing in the morning, they won't be able to wait until we get to a restaurant, so I want to have lots of munchies and breakfast items available.
Net Grocer's prices are a bit higher that a traditional grocery store, but the shipping price was not as bad as I thought it would be and it will be so nice not to have to pack all that up myself.
Right now, I plan to purchase umbrella strollers that recline so the kids can nap as needed as we go through the parks - the park hours will probably be short in October, so I don't want to waste so much time going back to the hotel to nap, when I know we would probably have to fight them to go to sleep anyway.
But- We will be flexible!
Have a great time - let us know what you learn about traveling with little ones when you get back!
Cathy