I would have suggested Anna Maria Island too, but they don't really have any family friendly resorts per say.
It's funny you mentioned Anna Maria, most people I tell about it has no idea where it is, it's such a tiny island.
Our family (grandparents, sisters, our kids, dh's, my mom and my aunt) has been spending a month on Anna Maria Island every year for the past 19 years!!! We used to rent one house every year for 18 years but last year found out the owners sold it and weren't renting it out anymore. So last year we tried a new house, it isn't quite as nice as the old house but it was definately bigger and we are renting it again this year. I LOVE Anna Maria!