Are they in the barn with other animals?
I'm sure its warm enough in there for them, the drafts are what you have to worry about.
I would still maybe get a box for each of them and fill it with some more hay, so they can get in there.
You could probably make some rice heating pads for them. Put them in the microwave for a few minutes, and bury them in the hay so they can't touch them because they get hot. It will help heat them, but keep in mind that when the heat goes away they could be colder than if they had no heat at all. Thats why my chickens down't have them, just bedding and hay.
You could put a fleece blanket in their hutches. Foo likes to get under her blanket when she gets too cold. Its cute.
You could also cover your hutches to make sure their body heat stays in the hutch and doesn't escape out of the wire sides, other than the floor(if its wire) Just cover it them with an old blanket or whatever you have that your parents will let you use.
Hope that helps. I only have outdoor chickens and they have 10+ inches of pine bedding with hay to burrow down into. They have little pockets they sleep in, plus each other.