If you're adding six-week olds to an existing flock, you need to provide a safe pen for them during the day so the flock can get used to the idea of them belonging. After a week of this, open small chick-size openings, about 5 x 7", (more than one) so the chicks can come and go but the adults can't get in. Keep food and water inside this pen (called a "panic room") until the chicks no longer fit through the openings (around ten weeks).
Makes sure there are no dead ends or places a chick can get cornered in the run without an escape. Provide additional perches in the run around three to five feet off the ground. This is important for safety.
Take the chicks indoors to sleep during this first week. They won't need heat.
As for sleeping in the coop, at six or seven weeks, they can start learning to roost. After the first week of being in a safe pen during the day, you can install the chicks in the coop after the adult hens have finished laying for the day, letting the chicks explore their new home. Lock the adults out until almost dark, then let them in. Referee to keep the chicks from panicking and to make sure they are safe. It will help if you have a perch away from the older chickens where you can place the chicks. Everyone should settle down pretty quickly.
After that, you will need to show the chicks how to go into the coop at night for the first few nights. After the big girls have roosted, get into the coop and coax the chicks in with a flashlight if the coop is darker than outside. Before you do this, it would help to teach the chicks to respond to a verbal cue using treats. It only takes a few minutes and they will learn. Reinforce this lesson off and on for one day. Use the same verbal cue each time you offer a treat.
If the chicks don't want to roost, let them sleep where they want. Again, no heat necessary. In the morning, after being in their panic room for a week, they will know to run into it.
