How do I get my dogs to stay in their doghouse?

How do I get my dogs to stay in their doghouse during the night? They both a 2 month, female, Labrador-Chow Chow mix and they're not potty trained yet. During the night they sleep in our front yard (its blocked off from the street) and they're sleeping on our doorstep, which is right next to their dog house. The doghouse has enough space for both of them and a blanket. I try to put them in their but they come back out. I don't want them to sleep on the cold doorstep but they're not potty trained so they can't sleep inside. Please help!