Since you have power, you can put a heating pad under a towel but yet where it can stay plenty dry and then put a cover over that. I did that with my second batch. I took some hardware cloth and bend down the sides, put the heating pad under a puppy pee pad and then covered the hardware cloth...
With that small of a coop I would be afraid of a fire or overheating them. They've got to have a way to get out of the heat.
I put my first baby chicks out in the coop (4ft. x16ft.) when they were about the same age but it was March. We put a thermometer on the floor where they were which I...