There are some great beaches in Naples if you can handle the drive there. It is about 2 hours from WDW, past Tampa.
I used to take my boys when they were younger and they loved it. Somehow there is one beach where you can pick up tons of petrified sharks teeth. They are black and very shiny so they are easy to spot on the white sands of FL. I don't know why they seem to collect on this beach more than any other, but they do. My boys used to get enough to bring to school for each class member. I would go in on a rainy day and we would do a craft of wrapping copper wire on the sharks tooth. I would bring in black cording and the kids would either make a necklace or keyring, depending on what they wanted. The teachers loved that I would take the class for about and hour and the kids loved getting a gift.
I wish I knew the name of this beach, I know the directions myself but could never give them to someone else. sorry. But if you drive to Naples and go toward the beach area I know someone could give you directions either at a gas station or shopping center. I had read about this beach somewhere and got directions myself once at a gas station. It is a State park so there are changing and bathroom facilities and showers to rinse off.
There are quite a few elders who do the beach squat there to pick up the sharks teeth and sell them to the shell stores.
Sanibel Island is a great place to pick up shells, but it is a little farther down south. There is a charge to get onto the causeway to the beach, about $7, and there are no quick type restaraunts that you can use the bathrooms. There are a few hotels and restaraunts but they don't like to let non guests use the facilities so you might have to get in the car and drive to a gas station once you are on the island when nature calls. I haven't been to Sanibel/Captiva for over 10 years though so this might have changed.
Also, if your kids are into shells, make sure you stop into one or two of the shell stores they have scattered around Orlando. My boys bought quite a few when they were younger and made showboxes for them. I still have them up in the attic, even though my boys are now 29, 26, and 22.
Wish I could help you more, sorry. Peggie
...I don't think I can go wrong with any of the beaches you mentioned. If I was thinking I would have given ourselves an extra week in FL after our WDW stay so we could visit all these fabulous beaches. I guess we will just have to go back again soon! 
). That's what happens when you are land-locked in the midwest, I guess.
