My family and I have been to Sanibel many times, and it's a beautiful and relatively quiet place to vacation. We always stay in a condo, but there are several very nice hotels on the island. We've had lunch at the Sundial Resort, and it seemed to be a pretty nice hotel.
As for restaurants, there are many, for the size of the island. With a young DS, I would suggest The Hungry Heron (very kid friendly and large menu and large portions), Matzaluna's (can draw on the paper tablecloth), Island Cow, Cheeburger Cheeburger (you'll need a big appetite here!), and if you venture over to Captiva Island, The Mucky Duck, and perhaps one of the most memorable restaurants for you and DS--The Bubble Room (gets very crowded, so I would suggest going as soon as doors open for lunch or dinner).
There are many more restaurants, and I'm sure you'll find some of your own favorites.
Things to do: rent bikes--bike paths across the whole island, go shopping!, Ding Darling National Wildlife Refuge, shell museum, golf, tennis, fishing, shelling, dolphin and sunset cruises, and just relaxing by the pool or on the beautiful beaches!
I would say temps are in the mid 70s during the day, 60s at night....I think spring break is a popular time, so the island will be busy, but still very enjoyable.
Just an FYI--in order to get onto Sanibel, you have to cross the causeway from Ft. Myers, and it's a $6 toll.
Hope this was helpful. Let me know if you have more specific questions about anything.
