I also would not recommend placing a plastic bag inside the sleeping bag. You can place extra clothing or blankets in the bottom of your sleeping bag to take up that excess room, especially for the kids who are smaller. That extra room requires body heat to warm it up. You can fold the excess under and place a plastic bag over that to remove that excess space.

You can get the military surplus sleeping systems for around $100, which includes two bags and a bivy bag that all snap together. It would easily have worked for those temperatures.

My scout troop was camping this last weekend also. We had low to mid 20s at night. I make sure that they all have a good insulating pad, not necessarily comfortable and soft, but good insulation. I also have a supply of military surplus wool blankets for those bags that may not be so warm.

Long underwear and warm socks do help inside the bag. For really cold feet, you can get a set of down booties to sleep in.