You're certainly getting very good views here, and not to discredit any of them, but perhaps you should get the viewpoint of someone who dives the keys on a regular basis.
First, and strictly IMO, renting a wetsuit is like renting your toothbrush, using some stranger's hair brush, borrowing your neighbor's underwear. So that being said, buy your wetsuit. As far as your 12 yr old, yes, they grow quick; my 14 yr old daughter grows like a weed, however, my 12 yr old boy, grows a bit slower, and things last him a bit longer. Nevertheless, if you're planning on diving often, you're more than likely going to end up spending as much on wetsuit rental for your kid as you would if you purchased his wetsuit, and by purchasing it, you're not giving up on the hygene factor. Check out diversdirect.com. They usually have good deals on kids wetsuits, and you should pass by one, maybe two of their stores on the way to the keys (Florida City and Key Largo).
As far as the temperature is concerned, here in So. Fla. most divers have a 3mm as standard equipment, and when it gets colder, usually a beanie hood and a 2mm vest does the trick. Just last weekend, I dove in 71 degrees with my 3mm, a beanie, and a 2mm vest, and the dive was 73 mins. Two years ago, however, while diving the reef known as dry rocks, where the statute of the "Christ of the Abyss" stands, I dove with just my 3mm, no hood, no vest, in 72 degrees and after 20 mins I was done.
If the water is 75 degrees, the 3mm suit, especially a new one, should do it. But, if in doubt, get a 3mm wetsuit with a 2mm hooded vest and wear the vest under the suit and you should be good to go. If you use the vest/beanie combo, the vest can usually be worn outside, if it zips in the front. Warm reef gloves (2mm) usually do the trick also, but leave the full foot fins at home; get some booties and open heel fins.
Just my .02 worth.
BTW, with whom are you doing your dives? One thing you want to keep in mind, in the keys, 3 - 5 ft waves will create enough surge to keep you busy while doing skills, even at 25' depth. Reefs like Molasses, Snapers ledge, and French's are very good for check out dives. Dry rocks is ok, but it must be dived at high tide, because if you dive it at low tide, you'll see why it is called dry rocks. If you dive dry rocks, DON'T TOUCH THE STATUTE.. it is covered with fire coral.
If you can, try and to the Bendwood or the City of Washington...two very old wrecks (I think 19th or early 20th century). If you go to the City of Washigton, there is a resident 300 lbs Jewfish that might just come and say hi.
Semper safe,
Rick
) you might want to dive in cooler waters. I think 5mm are more versatil, you won´t be hot in warm waters. 
