If they're in the house, I'd stop using the lamp NOW. I doubt your house temps. drop lower than 65 degrees at night time. If they're like mine, they'll squawk a bit and huddle together, more because they've never been in the dark before than because they're cold. So maybe make sure there's a nightlight near their brooder.
And depending on what you have available, I might consider moving their brooder into a garage (either with or without a lamp at night, depending on garage temps). The thing is, unless you're planning on waiting until outdoor temps are consistantly close to your house temps., you should be looking at ways you can get them use to cooler and cooler temps.