We spent a night at South Beach, which was pretty full on. Great sights and sounds but quite an adult vibe and probably misjudged that with younger kids, but not an issue for you. Architecture is iconic and it's a great night out.
I'd also recommend Fort Lauderdale - the segway tour is fun and there are plenty of beachfront resort type hotels.
I'd also head a little further north and do Kennedy Space Centre, if that is remotely interesting to you, it's great.
Our most enjoyable time in South Florida however has been in the Keys. Love the island life feel down there and lots of small B&Bs and great restaurants. We';ve tended to stay around Grassy and Duck Key where the islands thin out.