Since an Easter/Spring Break exchange into the southwest FL Gulf coast is so difficult, you may want to relax your standards a bit and broaden your resort list as much as possible. If Spring break access to warm Gulf waters, ocean sunsets, and white powder beaches is important to you, staying in simpler digs could turn out to be just as fun as an HGVC-type vacation. Depends on what you like.
The Marco Island/Sanibel/Captiva area has many of the HGVC affiliates. A little north of there, consider the resorts in the stretch from the Sarasota area up to the Bradenton Beach area (including Long Boat Key), or even up to Clearwater Beach or in toward Fort Myers. There are a number of smaller, older resorts which are generally clean/functional albeit rather dated. But a vacation at one of these still puts you in good general proximity to gorgeous beaches and abundant local activities, even if most don't offer much for resort amenities. Just a thought (often kiddies just want a nice pool anyhow).
(By the way, I've always thought that tiny photo of your cute little peach is darling!)