Thanks!
FS Surfside has been on my radar for a while but just seemed less little-kid friendly to me - in particular a bunch of reviews comment that the kids pool is in the shade in the afternoon (particularly important in Dec). But - if people disagree I'm very open to hearing that as I'd love to have an excuse to visit...
Ritz Key Biscayne - I couldn't tell if there was much for very little kids - but it did seem pretty strong as they get older.