Seven week old chicks in southwest Louisiana, they could handle it on their own without Mama if it got into single digits, which it will not. Don't worry about it being too cold for them at all.
My nests are not on the coop floor so I don't have broody hens sleeping in them with their chicks. Some people purposely put nests on the coop floor as they think Mama needs a place to take the chicks. We all do these things differently.
I've had a broody hen take her chicks to the roosts at 2 weeks. I've had broody hens not take their chicks to the roosts before she weans them. I've had a couple of different broody hens wean their chicks at 3 weeks, I've had some not wean the chicks until well after two months, closer to three. If I had to come up with a time, most of my broody hens take their chicks to the roost around 4 or 5 weeks, but each broody marches to her own drummer. They wouldn't know what a calendar is if you gave them lessons, let alone how to follow one.
I personally try to intervene with my broody hens as little as possible. I figure I do less harm that way, but that's usually with younger chicks. At seven weeks I don't think you will harm them if you intervene. I don't know what your coop or roosts look like. In my opinion and experience that can have a big influence on when they roost, especially after Mama weans them even if they were roosting before she weaned them.
I won't talk about my brooder-raised chicks, they tend to be a little different. I've had broody-raised chicks continue to sleep on the main roosts after Mama weaned them, usually in a far corner as far from the adults as they can get. I have a juvenile roost in the coop, a foot lower and horizontally separated from the main roost by a few feet. That juvenile roost gives them a safe place to go that is not the nests. Sometimes the broody-raised chicks sleep here instead of on the main roosts. I've had a few (not many) broody-raised chicks leave the coop entirely and try to sleep out of the coop after they are weaned. Older hens can be pretty brutal to younger birds on the roosts. Some are worse than others, they are all different, but some pecking is pretty normal if they try to sleep to close to the older hens.
If you are getting poopy eggs trying to get them to roost is probably a good idea. What your roosts look like and how many other chickens you have sleeping up there can influence how easy that will be. If I were not getting poopy eggs I'd just let them work it out. Eventually they will.