How great that you are giving them a warm place to take shelter.
I have an outdoor enclosure that we put cat doors from the house to the enclosure. We have had good luck with the cat doors sold at the home improvement centers like Lowe's or Home Depot if you have one nearby. A hardware store should have them also. The prices are better than at the pet stores.
I do not have a brand favorite.
To get them accustomed to going through the door I taped the flap open with a strong tape so they could walk through then put the flap down.
Keeping food, water and a litterbox in the garage will encourage the momma cat to stay in there out of the cold.
When the momma cat weans her kittens it might be a good time to get her spayed. She may likely become pregnant again and keep bringing you her babies. Best of luck to you.