The Keys are great (to visit at least...but I'm not sure I would be able to live there....it'* such a laid back environment I think I'd be bored to death). Isla Morada is a really nice little fishing community...people leave their doors unlocked at night, neighbors all know each other, etc; but the problem is the drugs and alcohol. It seems everyone down there (the Keys in general) has nothing to do but get inebriated...it may be a dangerous environment for your kids in that respect.
But on the other hand, there is little other trouble for them to get into...as willwren said, there is little crime. And the area is beautiful...paradise I'd say. The week I spent there last year was one of the most pleasant weeks of my life. I'm someone who has trouble dragging my butt out of bed at 9:00 am, but there, it was such paradise, that I woke up refreshed and ready for the day at 6:30 am (without the help of an alarm clock...just the sounds of the rainforest outside my window).
Oh, and it'* hot
...the humidity is so high that it feels like 90 degrees when it'* only 70 outside. But OTOH, it is always sunny and beautiful...the frequent Florida rain never seems to touch the Keys (except during hurricane season).
Keep doing your research, and if you need any answers to specific questions like cost of living, etc., PM me and I will ask my friends who live there. I hope this helps a little.