The Loews on South Beach has a good kids program and the hotel seems to be trying to please familes.
http://www.loewshotels.com/aboutus_programs_kids.asp
Fountainbleau is a good choice as well. Its got a fantastic pool with a big kiddie section.
All the Ritz Carltons have a good Kids program, although I personally would not recommend South Beach. Too much effort is spent at that hotel to be trendy,hip and upscale dont go with strollers and diapers. The Key Biscayne property is probably the best choice if you want to do the Ritz as its located on a nice beach that has shallow water (like 1-3 ft deep) for a long ways out. The property is also located near Crandon Beach which has a great kids playground with water games, a wooden horse carousel and more.
If you stay in Miami Lars, take the kids to the Miami Childrens Museum. In July, its an airconditioned haven for the little ones. Ask Pierre (okidoki) about it. Sharon and him took their daughter there when he was in Miami last year and it was a big hit.