I'm meaner than most, I guess. Our first year with chicks I couldn't handle the mess, the dust and the noise for one more day! So when they were 5 weeks old I shut off the heat lamp in the brooder, cracked the window in the room they were in, and listened to them scream their protests when it got dark in the room that first night and they didn't have their light on. By 5.5 weeks, on April 1st I evicted them out to the still-unfinished coop. Temps out there were in the 20s and dropping. I put a heat lamp out there and then as the temperature kept dropping that first night I'd get out of my nice warm bed and check on them. They were snuggled in a pile of beaks and feathers in front of the pop door - nowhere near the heat lamp. The second night it was the same story. The third day the heat lamp came out - they didn't need it so I wasn't going to risk it. That night it snowed. And we continued to get snow up until our last snowstorm on June 6th. They did great. If I'd waited for perfect temps here in Wyoming they would have been laying eggs in the brooder!
Now, like @Ridgerunner and many others, I brood outdoors from the start using the heating pad method RR mentioned. As long as they have a place to warm up - in RR's case the side with the light, in my case the heating pad cave - they do fine. They actually need that cooler area to encourage fast feathering and acclimate to the environment they will spend their lives in, and with no light glaring all the time they learn that once the sun starts going down it's time to sleep. They don't sit under a broody hen 24/7, so why do we heat their entire area and think we're doing better than Mama Hen?
At this age your birds are more than ready to go out without a heat source, but I do understand your reluctance. Personally I'm much happier now raising real chicks instead of dependent little Divas, and all I do is duplicate Mama Hen as closely as possible.
Edited to add this:
Edited by Blooie - 1/21/16 at 7:23am