You should turn off the heat source during the day and see how they do. At that age mine are already out, with out heat. however I never put young birds with older birds. For really young birds, I start them in a temporary fence. It is a 20 foot fence I just close the ends together and make a surround. I add a cardboard box, food and water. I start this early, and during the summer I start this at a few days old. I supervise and usually am working in the yard. I have a hen and a drake Muscovy who love baby's and they usually set by the fence to help watch. You can do the same. Take them out on sunny days in a fence away from other animals. They have the box to get away from the wind and sun. Start an hour a day and build till it is all day. Supervised. I do not put baby chicks I have hatched in with the general group until they are pullets and I know I am keeping them. I do raise them on the same grasses if they are going to stay with the flock. If they have hatched out under a hen they are with the general group from day one. Cross contamination is ruff on a flock. Your chicks can bring in new molds, bacteria, etc into your established flock and vice versa. So go slow. A few minutes a day to build immunities. Be observant and in a month you should be good to go.
