I have to disagree with a couple of things that have been said on this thread.
First of all - concerning "adult" activities. I do not see it as a problem. The Keys are 100 miles long, and as far as I can tell, only Key West has a reputation as a party town. And even at that, you can avoid the worst of it by seeing it by day. I guess there is a carnival or festival or some such thing there at some point, and you'd want to be sure to avoid Key West at that time with your kids. But to avoid the whole area (The Keys) because of this would be a shame.
Second - concerning prices. I just haven't found it to be true that everything costs more in the Keys. Gas is comparable to most places on the mainland. There are enough chain grocery stores (Winn Dixie, Publix) that most prices are reasonable. Some things, like produce, might be a little high. But most things are not much different than in the mainland.
To me, the main reason to go to the trouble of going to the Keys is to see the reefs. So that's back to the snorkeling - or if you're certified - the scuba diving. You just can't get anything like those reefs on the mainland.
Just my 2Cents.
-Speak