I'd do Miami Beach. I just got back from a long weekend on South Beach and saw a lot of families with children. The beach is nice, and if you stay on Collins Ave or Ocean Drive, you can walk to everything you need. There are plenty of resorts to choose from, though I prefer condo rental to have a kitchen (at least for breakfast) and some extra space.
Delray Beach is really nice. You can Fly in to PBI or FLL. The main downtown road is called Atlantic Avenue, and there is a Marriot right on the beach there. There are a few other hotels too, that one just comes to mind. There is a ton of restaurants and shops on the ave, all within walking distance. It is a really cute downtown area, that is much less overwhelming than Miami or Ft. Lauderdale, but still plenty to do.