The Brevard zoo in Melbourne has a neat kids water play and playground area for toddlers and young kids.  There is also the Indian Riverside park in Jensen Beach.  There you will find a big fountain for the kids to play in, an intercoastal beach area for the kids and adults, and even the Treasure Coast Children's museum that's currently expanding.  Depending on where on the Treasure Coast you live, the Minsky Gym in Port St. Lucie has Toddler Tuesdays for ages 5 and under and MY GYM has fun classes for babies on up.  I'm currently looking into the MY GYM for my kids.  I have a 3, 2, and 5 month old and they desperately need some social interaction.