I hope I can answer your questions well enough here!
1. Sunpass does save a lot of money on toll roads and since you're going to be driving all over the state, it might be a good idea to use one. Otherwise, they charge you by license plate and I'd assume that you'd have to pay the tolls when returning the car. Each toll can be 50 cents to $1 cheaper. I don't believe I've ever seen a rental car that included one. They may sell them in the airport, but I know you can purchase them at any Publix store too.
2. I'd suggest sticking with the big rental car chains such as Hertz, Enterprise, Alamo, National, etc. There will be no-names out there, but their policies could be a bit sketchy.
3. Those are all great places to take families. Disney is great for all ages and I'd recommend going to as many of the parks as you can. If you have more time in Orlando, check out Universal's parks as well. Your kids should enjoy the white sandy beaches of the West Coast when you're in Siesta Key. I personally like the West Coast's beaches better than the East Coast's. Definitely take an airboat ride while in the Everglades and make sure to put on tons of bug spray. I haven't been to Kennedy Space Center in over twenty years, but I'd imagine that could be entertaining. The only other place I'd maybe bring up is a trip to Sawgrass Mills if your teens like to shop. The place can be chaotic at times as it's always crowded and over a mile long.
Enjoy the Sunshine State as there's no shortage of things to do. Just remember, it's brutally hot down here in the summer, so wear lots of sunscreen and drink a lot of fluids.