Most don't place them outdoors permanently until they get their juvenile feathers - 5-6 weeks.
Others brood theirs totally outdoors - with our without momma hen.
@Blooie has a thread on using a heating blanket for this - more like momma. post #1 . It would allow you to move yours out...
glad you have joined us.
Lesson learned: don't think the folks working at the feed store really know anything about chickens. .
If the little one is eating, pooping and acting normally, then IMO you are doing a great job.
I would make sure the brooder is large enough to allow the older...