Well, first of all, make sure both you and your friends don't underestimate the heat. Drink lots of water (some people will also freeze towels the night before and bring them with them), so you don't get dehydrated, wear loose clothes, and you might consider bringing some deodorant. Trips to the pool and water parks will certainly be helpful. Especially because you're used to the heat, you should be fine though.
In terms of crowds, you'll be catching the tail end of the peak summer months, so while it will still be crowded, it will be less crowded than it would've been in July or June. Personally, I travel every year to WDW during the summer (Fourth of July in particular) and have never found the crowds to be unmanageable (like they are during NYE week, during which I've also traveled). Fourth of July is a bit crazy, but outside of that, they're not bad, especially once you head into August. Don't underestimate the crowds though; many are surprised by how crowded the parks are getting these days. The good news is that your trip falls early enough in the summertime that you should still be able to take advantage of later park hours (MK open till 11pm or midnight, etc...), which is one of my favorite parts of traveling during the summer.
Because you have younger kids, your best strategy will probably be rope drop. Show up 40 minutes or so prior to posted park opening and go around the parks until you get kind of tired (probably right after lunch!) then you can head back to your resort for the hottest part of the day to nap or rest by the pool and come back in the late afternoon/early evening to grab dinner in the park, see the nighttime spectaculars, and ride some more rides. This way you take advantage of lower crowds and the cooler times of the day (Florida summer "cool" is high 70s btw). you can swing it, you might consider opting for a 5-Day theme park ticket so you can visit MK twice (if you purchase through a site like OfficialTicketCenter.com, the 5th day is free and often cheaper than buying through WDW).
Another potential strategy would be to sleep in, show up later in the day (like 2pm), use your FP+ options then, and stay through park close to take advantage of lower waits. This works better for groups with teens and is not as ideal during the summer, but whatever you choose, you're going to have a great time. If you can swing it, you might consider opting for a 5-Day theme park ticket so you can visit MK twice (if you purchase through a site like OfficialTicketCenter.com, the 5th day is free or a minimal cost), no matter which strategy you go with.
In terms of a beach day, I'm not sure if you had one in mind, but the closest one to WDW is Cocoa Beach near Cape Canaveral. Don't get me wrong, Cocoa is very nice, but if you have a little extra time, you might consider heading west to Clearwater Beach, which is absolutely gorgeous and the water will be rather warm and gentle (it's the Gulf of Mexico after all) that time of year. Also, check out the Beach Shanty Cafe. It serves an awesome breakfast near the beach and it's very, very inexpensive. Otherwise, check out Clear Sky near the beach for dinner. Another awesome restaurant.
Personally, I find summer to be my favorite time to visit WDW, so I think you're in for an awesome trip!
