I've been there in August and I can say that it was hot and humid, though I still had a great time
diving. Either way, don't let the weather put you off as there's almost always a nice breeze throughout the day and nights aren't too bad. Mu advice since you have kids is to take it easy during the hottest part of the day (12-3pm).
By the way, don't buy into the saying that the northern part of the Keys is cooler...no matter where you stay in Florida Keys it won't make a difference as it will still be hot and humid.