I stayed at South Beach after a cruise in 2004. I stayed at one of the nicer hotels on the beach a few blocks away from Ocean Drive. I do not particularly recommend staying on Ocean Drive if you have small children. It is the happening street on South Beach with nothing but bar/club after bar/club after bar/club. I would imagine that staying on Ocean Drive would be quite noisy and rambuctious (sp?). It is one big party scene where the young, not so young, the beautiful and not so beautiful go to see and be seen.
There are plenty of hotels along the street that lines the beach a short walk from Ocean Drive. South Beach is plenty safe for your family but there is limited things to do if you are not into partying and drinking the night away.
When picking a hotel, read as many hotel reviews on the web as you can. Many of the hotels on South Beach are older and since the purpose of South Beach is to party, a lot of the smaller hotels are not in the best condition (moldy, musty, bad service, rundown). For the most part people don't go to South Beach for the hotel. They go to party and then crash in their hotel room.
After reading loads of reviews, I settle on a bit more upscale chain hotel which was lovely. If you want, I can track down the name of it.
Depending on your budget, you may do better staying at a Biscayne Bay or Fort Lauderdale hotel on the beach. I am sure it is a more family oriented environment.
(cruzin' solo by design)
Freedom ('09), Imagination ('08), Victory ('08), Destiny ('08), RCCL Navigator ('08), RCCL Majesty ('08), Glory ('07), Liberty ('07), Fantasy ('07), Destiny ('06), NCL PofAm ('05), Fascination ('04), Victory ('04), Fascination ('04), RCCL Voyager ('03), Fantasy ('99)