Oh what fun!!!
I imagine you have already gone through old posts regarding heat. There are lots of good tips there. Some I used with Evan when he was little was to carry a spritzer bottle. They even have them now with little fans on them. Also if we went out and about and I was worried about the heat, I would put several ziplock bags with wet washcloths in them right down in a cooler of ice. Using the ziplocks that way, we could carry our drinks or whatever and use the washcloth around his neck or pulse points to cool him down when needed. One more piece of advice, alternate your outdoor activities with shows and stuff that are air-conditioned, and or plan those indoor activities during the hottest sunniest parts of the day. That will give all of you a cool down and rest activity
Evan has shown me he is much more tolerant of heat than I expected when he was little.
I guess my biggest advice is HAVE FUN.