We'll be there late July and these are some of the items I'm packing in my backpack...
1) Hand Towels - Going to wet them and place them in our hotel fridge overnight. This way they'll keep our water bottles cool and will also be cool when we use them. Plus we'll keep on getting them wet when we see a fountain and place them around our necks as needed.
2) Portable fans - We got one neck fan and a couple of handheld fans. Also bringing USB portable battery packs to recharge them and our phones mid day.
3) UV Umbrellas - Picked up three Totes UV Umbrellas that are UPF 50+ and 55" that should be enough to cover 2 people. Part I like about them, is they're compact and lightweight for when I'll store them in my backpack. I recently tested them out at my son's soccer tournament and they worked great!
4) Plan for breaks - Especially around noon to avoid the heat and crowds. You can go back to your hotel to rest up but if you are staying in the parks, plan for some activities that have shaded areas.
For Disneyland, I'm planning to mobile order lunch early morning for the Hungry Bear at 11:30am and have reservation at Oga's at 12:45pm. Then planning to ride the Disneyland Train. That should keep us shaded for a couple of hours till mid afternoon.
For DCA, I'm planning to hit both Mickey's Philharmagic, Animation Academy, Monsters Inc and Soarin ride ques right after lunch at Pym's Test Kitchen. That should keep us shaded for a couple of hours till mid afternoon.
5) Memorize the Water Bottle Refill Station locations. There are 6 of them in Disneyland and 2 of them in DCA (as far as I know, there may be more). So always hydrate drink a lot of water throughout the day and fill your water bottles as you pass near those areas.
Here's a good video to help with locations in Disneyland...
In DCA, there's one in Avengers Campus and I believe on near GRR\Soarin area.
Plus you'll be able to get water from any restaurants in the parks.