That's a common error. Most of us that have been through it recommend having the area ready before you even get the chicks. Life has a way of getting in the way of finishing things. When it is time for them to go out, it is time.
By 6 weeks old they can handle temperatures down to freezing or below as long as they are out of the wind. You have no worries there. By 6 weeks I'd have the pop door open every day and let them decide if they want to be in or out. But mine would be sleeping in the coop. Do you still have...