We actually just got back from Naples a few weeks ago. We stayed with my parents, and it was me, DH, and 5 year old son. DS had a blast!!! IMO, the best beach is Naples... we tried Marco Island and Sanibel, and they didn't compare (IMO- no offense to anyone). The Travel Channel actually rated Naples beach the best beach in Florida. There is also a small water park in Cape Coral (about an hour's drive- called Sunsplash family waterpark) that my son loved and we went to twice.
The beaches in Naples are great -- warm water, nice sand, and lots of shells. (if you go underwater about waist deep, you can find some sand dollars under the sand too!)
Weather was hot (but it is summer....) about 92 deg. every day, but really didn't bother us at all.
We plan on going again next year!
Let me know if you have any other questions!
Jen