I do believe in the benefits of brooding outdoors and I would love to. How would you transition them. This is my coup I have solid inserts that cover the window to reduce drafts on windy/ snowy days. I had thought of putting the heat lamp in and throwing them in. But they are so small only 3 days old. It goes against everything I was taught growing up on a farm
You need a heat source. What have you chosen? Heating pad system is what I strongly recommend.
How about adult chickens? You need to rig a chick safe pen if you have an adult flock also using the coop.
You say all drafts have been blocked off. How about dampness? Is it protected from rain or snow flying in a wetting the chicks?
If you have electricity to your coop, there's no reason not to rig up your heat source and move the babies in.