I guess I'm not sure what 'Affordable' really is -- what's affordable for you might be totally out of my price range. With that said, I'd definately look into Key Largo. There are lots of flights and even a few airports you could choose between to save money.
Several people have suggested Bonaire, which is supposed to have great/easy diving, but when I had just been certified shore dives were the last thing I wanted to do. If you're whole family is new to the sport, it's nice having DMs around on the dives to help make sure things don't go wrong (Especially with an 11 year old). Shore diving is also extremely tiring, as it involves more work setting up and breaking down your gear, finding the dive sites, hauling your gear out to them, suiting up, fighting any waves there might be, etc. (Bonaire could be different -- my only shore diving was in Catalina and it was exhausting!)
Hope you find what you need. Also be careful of going to the popular spring break places for the universities -- unless you want to be on a beach with several hundred drunk college kids.