I brood mine outside from the start. They thrive on some cooling, rather than having a hothouse environment, so if you can provide them a space well away from the heat lamp to frolic and play, they'll be so much the better. Here in Northern Wyoming our temps are getting up into the 60s during the day and still in the 40s at night, but my Littles are fine with that - I've brooded with temps in the teens and twenties with a snowstorm or two tossed in just to keep things interesting. I don't use heat lamps...those things scare the pee-wadding outta me, and they heat everything - the air, the walls, the floor, the bedding, the water, the food - when all that's really necessary is a spot for chicks to go warm up, just like they would under a broody hen! So I form a "cave" out of scrap fencing, put a heating pad over it, then a towel and layers of straw. They love it!
I'm assuming by now you've put them out and they are doing well. I wish I'd seen your question when you first posted it so I could be more helpful.