You can also warm bricks up in an gas oven or on a grill. To about 150 to 200o , then wrap them in foil and a towel and place them in/under brooder. Just be sure the little ones can't come in contact with them directly. Depending on how cold it is you may have to rewarm some a few times though the day. I've done that it helps.
Oops, placed my post in the Michigan thread. I've used those hot pads that are for sore muscles, found in the pharmacy section of most grocery stores, the large size. I put something over them, a blanket or towel, because they get quite hot.
Thermacare packs from the pharmacy stay warm 8-12 hours. You could also fill a 2 liter bottle most of the way with hot water and put lid on. They will huddle together. Keep them covered to keep the warmth in.