My family went to Sanibel every year when I was growing up. The last time I was there was about 10 year ago, before hubby and kids. My family loved it, but it depends upon what you are looking for. It is isolated--only one causeway back to the mainland. There is a grocery store there, but it is also more expensive than on the mainland. My family always wanted a get away in nature. There are no highrises, relatively few stores, bars, etc, and lots of parks and natural areas. If your idea of a good time is going for a nature walk, a swim, a bike ride, and a boat ride, then this is the place to go. If you want shopping, an amusement park, and some bars, this is NOT the place for you. Beaches are gorgeous--and yes, with a lot of shells, so the point about kids needing water shoes is right on, but little ones love to collect shells!
You will pay a bit more for accomodation and groceries on Sanibel, but in my experience, if you pay to stay in a nicer condo, you save a lot of money by eating in and staying on property for a lot of your entertainment. Fall should be low season, so I bet you can find a deal. There are lots of condo complexes right on the beach, so I'd look into several. We stayed at a couple of different ones, I don't remember names, but most every place there is nice. Look for a complex with the ammenities you will use, but not the ones you won't--e.g. pool, tennis, golf, shuffelboard, bikes for guest use, etc. Whatever you decide, enjoy your vacation!