There are a couple of ways to keep baby birds warm without power, but in my opinion, mimicking how they would keep warm in the wild would be the best way. Make a nest out of blankets, paper, or straw bedding (you can also use a heat pack or some hot water bottles, as
@Lady Jane has said, in or beneath the nest). Then, put the nest in a sheltered area (it should be the warmest area in your house, if possible) and put the chicks in it. They will naturally huddle together and keep themselves warm.
I'm wishing you good luck and I hope everyone stays safe!