Does the coop have good draft protection but some ventilation up high? Have you done anything to acclimate your chickens, get them used to cooler weather? How damp is the weather too? Really misty humid air can be a bit of a problem but if it is dry they can handle it better.
My brooder is in the coop. I keep one end warm but let the far end cool off, really cool off, if the weather is cold. I’ve had frost on the far end some mornings when it really gets cold. The chicks play in the entire coop, even the colder end once they get to a decent size, just going back to the heat when they need to warm up. I’ve had chicks raised this way in that brooder go through colder nights than you mention at under six weeks of age with no supplemental heat. The coop they were in had great draft protection but good ventilation up high.
If yours are only used to tropical conditions, I’d be a bit nervous about just throwing them out there at that age. They should be fully feathered by that age so they should be OK, but is there any way you can expose them to cooler temperatures during the day for a while to see how they do and get them more used to the temperatures? If they huddle up they are cold. If they spread out and run around, they are fine.