She'll keep them warm. The first few days they'll all stay in the nest and then they'll run around with her and occasionally she'll sit and warm them up.
They can take considerable cold.
I recently had chicks hatch in that temp and I was surprised how little time they spent under her once they started going outside.
I think people coddle birds too much.
The sooner they get used to the cold the better.
If I brood without a hen I have a big elevated brooder with a couple lamps that have hot spots and lots of cool space. They spend more time in the cool areas than by the lamps.