The upper FL keys is a great place to dive with kids. The currents are generally mild (however they can get strong) and the dives are almost never great than 45feet with a lot of dives in the 20-30foot range. The marine life is excellent with an abundance of fish, and the coral is beautiful.
The dives are all boat dives so that adds some expense. However I enjoy boats, as do many. The downside of the Keys in general is the lack of nice beaches. However there is no shortage of activities to keep the whole family busy. After a day of diving generally lounging around the pool, getting some good eats, and relaxing in the condo is enough for me, but a trip down to Key West for a day off is a good time as is a trip to the everglades.
Enjoy,