I'm a FL native, and it will be very hot and humid in August, but you'll be fine. It's FL and gators live there so you might see some, but they do not hang out on the beaches since they are freshwater animals. (FL does have crocs in the Everglades, but you're not going there I don't think.) However, do be alert when walking around lakes and freshwater, but you will probably be fine.
As for beaches, it depends what your idea of the perfect beach is. Sanibel, Captiva and Naples are wonderful, quiet and relaxing beaches. They are not 'marshy swampy mess' . We natives used to tell 'foreigners'...this meant anybody but a FL native...that they were awful to keep them quiet, but the secret has long been out.
meadow 6....it's America...don't worry about the kids...Depending on their ages, you might want a suite, mini-suite or just a double room with a 'loveseat' or 'sofa' that turns into a bed. In the US, they don't have family rooms....almost every place accepts kids. You'll just need to tell them, 2 adults, 3 children and they'll help. Have fun!
Clearwater, St. Pete, Ft. Pierce, etc., are commercialized with amenities, but depends on what you are looking for. The best party beaches are Miami Beach,Cocoa Beach and Daytona. They're all great....just different.
Remember the Gulf (west)has calm waters (when there's not a storm approaching of course), but the Atlantic (east) coast has real waves for some surfing, body surfing, etc. Sanibel and Captiva good for windsurfing.