Check out Shell Vacations, also VRBO and HomeAway.
We found a link to a bunch of houses on Captiva in a small quiet neighborhood of similar stilt homes called Captiva Sunset that has a community pool and wooden walkway to the beach but is also walking distance to Andy Rosse Lane (small group of restaurants & shops on Captiva)
http://www.sunsetcaptiva.com/ Went there with MIL and 18 month old and rented baby gear. Flew into RSW.
Would have also been fine with a villa in South Seas or an oceanfront condo in one of the developments on Sanibel. Those actually had some pretty good deals.
We went to Marco Island last year and I have to say the Marriott resort was pretty fun for a 3 night stay, I'd have to have a kitchen with a baby around though.