20's at night in a coop is fine if they are well feathered. From the photo they look to be feathered out. Do you have a transitional area to put them for a few days like a garage? Acclimation to colder is more important than how cold it is outside. It's the shock of still being under a heat lamp to outside that bothers them. Turn off any heat source if not already, if a unheated space like garage put there for few days then outside. Stage down into the temp they are going to instead of heat lamp to cold all at once.
I'm North of you and have not had chicks out this early before. We have a 50F day coming but next 5 days still have teens for lows, not out of the woods yet here by Canadian border. Have had them out late April when you'd still get high 20's at night now and then and high 40's some days 50's others. They did fine. We currently are coming out of the cold snap of last 3 days. Check your extended forecast and ween the birds in prepration.