I took my DD when she was 2 before we knew she was heat intolerant but I still took extra precautions because I did know she was prone to dehydration. Look online for a clip on fan for the stroller- one with flexible blades that can't hurt little fingers. If you have a fridge in your room, freeze water bottles and bring them with you to the park- they will melt and be drinkable throughout the day but, while frozen, they are also great for your LO to cool off with. I also brought a water misting fan to mist her while we waited in line. Dress LO very cool- tank tops, lighweight cotton, sandals or flip flops since he/she will likely not be walking much anyway! Take frequent "indoor" breaks. Many attractions are indoors and air conditioned or, you can always pop into a counter service restaurant for a water/snack break.
I am taking my DD this August and now know she is heat intolerant so, again, I will be taking extra precautions! Even though she is 7, she will be in a stroller, with a fan. Lots of water- in fact, I am hanging a hydration pack from the back of the stroller so she can sip water easily. Again, dressing her cool is key. There are also areas for the kids to splash around so take advantage of those! And, if you're doing an extended stay and have time, take a mid day break and head back to your hotel for a swim or just to nap in the cool!
It is definitely hot there in August- I won't downplay that. But, with planning, you can keep your LO fairly comfortable and make sure the trip is the dream you hope for!