We just spent a week with my 9 year old and 11 month old. My 9 year old was the bigger complainer.

Some things we did:
-Use the baby care centers! The changing areas are the best, and it was a nice stop in AC for all of us to take a break.
-A good baby carrier is a must. We got a new one that was lighter because of the heat, the baby would nap in it and we could go on rides all we wanted.
-Make sure the baby stays hydrated. At 3 months I would ask the ped about adding water, at 11 months we had a sippy cup and she got a drink break when we did (on top of her formula).
-We bought 2 gallons of water and would fill the bottles before we went and had a powder formula dispenser filled. We took an afternoon break so I would carry 2 bottles and the formula and reload when we went back in the evening.
-Hotel breaks!! We never did that before, but with the baby it was a must. We went to the pool which the baby loved and it was a nice way to cool off.
-A good stroller. We got a new one for the trip, inexpensive but what we needed. We also got a side mesh pouch for the stroller that we put our hats and sunscreen in.
-A rain cover for the stroller. And use it. We brought it, didn't use it, and had a very wet baby until the heat dried us all.
Have fun!