Good morning Mags - The girls will be fine. You are a good chicken mom.
There are many different ways to configure the heat in your coop, but this is what I do:
I have hardwired a light fixture (ceramic socket) into the wall about 2 ft. above the roost. I did this because I was worried about falling lamps/fire, but a regular brooder lamp fixture will work just fine. Just make sure that you REALLY secure it so it doesn't fall and catch your coop on fire. Put it over your roost so that it will warm them while they are asleep/inactive and the temperatures are the coldest. Into the fixture, I have a 250 watt ceramic heat emitter, the kind used for reptiles. They can be purchased at large pet stores, but I bought mine on
ebay for about 50% retail. They are quite expensive, but worth it (IMHO). They provide a radiant heat - it heats surfaces instead of air - so it doesn't really raise the temperature inside the coop so much, but it will warm the girls while they hang out on the roost. More importantly, it does not provide any light, so it won't disrupt their sleep cycles. I started out using a red heat lamp (probably the most common source of heat used by BYCers) but found that it kept them up all night and tended to make them cranky/aggressive with each other. Anyways, I leave the heat on 24/7 from November through April. I also have a heated water dish (purchased at Mills Fleet Farm) to keep the water thawed. Very happy with that as well. I really didn't notice a sizeable change in my electricity bill last winter, and it was a real doozy of a season with record cold temperatures.
For that 3-4 week period in January when it gets really really cold around here (we're talk'n -20F and more), I supplement with a second heat lamp. I just use their brooder red lamp for this. While it does tend to keep them up (they party into the night because the lights are still on...), it just becomes so darned necessary to warm up the air inside, and the red lamp does a great job at this. It's only temporary, and they adjust fine - they're chickens... Just to give you some perspective, my coop got down to -5F inside last winter - with insulation and heat - and we made it through with no frostbite issues.
I gave my girls cracked corn for a bedtime treat last winter. They loved it and it will keep their bodies working hard overnight to metabolize (therefore creating more body heat).... B.U.T..... I think I managed to throw off their nutritional balance as they started feather picking (for protein). It was very stressful to me -- so, if you choose to give them carby treats, make sure to do it in moderation.
Hope this helps. Please feel free to PM me anytime if I can help you more. And don't worry... chickens do great through the winter if they are given just the minimal amount of care and attention. Make sure that the coop is dry and draft free, the water is fluid and then consider your heat.