Chicks hatched mid winter. Six chicks died overnight 😢

Mumma hen nested outside the yard in the long grass and we thought she had died. She returned late yesterday afternoon. We secured a chick proof cage today for her and the remaining chicks today.
Last night mumma hen hopped into a nesting box and the chicks couldn’t make it in and died from the cold. Middle of winter. Drops to 2 degrees Celsius overnight. She lost six chicks. She’s in the cage without a proper nest do they can get under her on the dirt and stay warm. We can put a nesting box in when they’re older and we’re sure they can climb on and out.