The chickies will grow faster than you realize.
There are a few people who brood chicks outside with a heat lamp.
The colder the weather the more steps you have to take to insure that the chickies will be warm enough.
What kind of nighttime temmps are you having?
I'm asking about outside in the coop because that is what my Dad and his mom (my grandmother) are saying to put them out in the coop with a heat lamp
(both have raised chickens)
Not sure of night time temps. I'm in western PA if that helps.
I don't want to come between you and your Dad and Grandma!
Check out this post from Henhaven. The lows in her area is 40s and the hut doesn't have a roof though its in a sheltered area. Its a great design. Her chicks are the same age as yours.
Maybe someone in PA, or somewhere with similar temps, can share what they do?