Are they in with the rest of your flock or separate? I had this problem with some 5 week old chicks. They were basically feathered so heat wasn't a problem, but mom stopped protecting them and in fact was joining in with the others bullying them. They got moved into a separate pen so they can grow out. It was my first batch raising chicks under a hen and I thought she was supposed to help integrate them, not make the bullying worse.
If they are separate from your other birds then you are just worrying about heat. If the chicks are peeping loud and seem unhappy, like they are trying to snuggle with mom for a warm up and she won't allow it, you could add some supplemental heat via a lamp (carefully attached in a place where they cant bump or touch it) just in case. But I bet they are fine. Does she sleep with them at night?
Depending on where you are located, in this weather they may not need that much heat from her during the day anyway. I wouldn't worry until the chicks seem unhappy or she actively chases or pecks them to keep them away.
