It depends on what you mean by a safe warm place, and if this cat is outside, either in a barn, shed or? If at all possible please bring her and the kittens indoors. You need a large cardboard box with sides high enough that mom can jump out, but the kittens won't fall out. Soft bedding, and a quiet room is best, you want to keep all other animals and people away from her as she acclimates to the family. Watch them closely and make sure all the kittens are on a nipple and one is not being pushed aside. Watch her closely, and 24 hours after the kittens are born, take the whole family to the vet for a check-up.
Also please, once these babies are weaned down the road please get your mom cat spayed.
If she is outside, even in a building, you run the risk of a Tom finding them and killing her babies to bring her back into heat so he can mate with her again. Trust me, you don't want that, so you need to bring her inside if she isn't already.