Should I put a heatlamp in there at night?
WARNING WARNING!!!! Use estream caution when putting a heat lamp into a coop!!! To many fires happen when pine dust gets into the air or a bird knocks the lamp down.
If the birds are fully feathered don't worry so much unless it gets VERY cold. If the hen is raising them she will do the work. If you are worried run a cord to the coop and place a heating pad on low wrapped in a protective blanket to keep them from direct contact and place on the floor of the coop. Do not cover with bedding, just that blanket or towel cover. If the chicks are big enough to get to the roosts insure that the roosts are no smaller than the flat side of a 2"x4". They will hunker down on the roosts and protect their feet.
Most of all. Don't panic. Those flighty rascles are hardy. My chicks survived a F1 tornado at 3 weeks old hunkered under momma.