They need the heat source until they are feathered out enough to take the nighttime temperatures. Most of us use a light for that. If you use a heat source that does not provide light, I think it is a good idea to give them light the first couple of nights so they can eat and drink when they want. I think that helps them get over the travel stress.
If Mama raises them, she has no way to provide them light at night. They don't need it. If they are used to having light and you turn it off, you will probably hear a lot of distress peeping. Chickens of any age hate change. They will probably protest greatly. They will get over it. It will not hurt them. But they are very likely to protest.