Second Sanibel/Captiva, lots of nature stuff to do and beautiful sand and shelling and blue blue water. I have found that anywhere north of Ft Myers is iffy for beach weather during the early spring or late fall, unless you prefer tennis and walking on the beach to swimming.
I am looking into Naples, Fla. for that reason, but the selection of housing is a problem, it skews very high end.

Sanibel Island. Lots of seashells to hunt for, nice restaurants, ver liad back and kid friendly. Easy to find a nice condo with a pool by the beach.

I like Destin Fla. White powdery sand, clear water, lots of condos right on the beach.

Gulf Shores Alabama is good too

Our son's spring break this year is the first week of April. We're looking for a not-too-fancy beach for our 5 and 3 year olds that will be warm that time of year. Flying outside the country that time of year is too expensive, so we are thinking of somewhere in Florida. We are considering Jacksonville and Amelia island, but I'm not sure it would be warm enough and if the waves might be too big for our kids. Perhaps the panhandle or the islands around there if the water is calmer. Any recommendations? We aren't really into resorts. Ideally we'd like a hotel or condo/cottage in a small, low-key town with decent restaurants and a few things to do other than the beach (for variety). Spring break is fast approaching and we have no idea where to go! Suggestions please! Thanks.