My 7 week old Ameraucanas have been outside for 3 weeks now and they're very happy out there. Temperatures range from slightly above freezing during the day to slightly below at night. I hung a 250 watt heat lamp in the insulated coop and gradually raised it every few days. (I used a chain plus two clips for extra security.) They spend their days out in the run and as they've gotten acclimated to the temperature they've pretty much stopped going back into the coop to warm up.
I also put my 3 week olds out there a couple of days ago. I partitioned off a section of the coop for them with wire, and they have the lamp for warmth. (I won't let them out into the run until they're bigger and more feathered.) The older chicks can still get to the warmth of the lamp because it's just on the other side of the wire barrier, but they often don't bother any more.
I have a digital thermometer with a long wire probe so when I'm standing in the feed room (which is where my adult birds sleep these days) I can see what temperature it is under the lamp in the coop. I also pay close attention to the behaviour of the babies to make sure they're comfortable. They do seem to toughen up pretty fast.