A foot tall. Could they be around ten weeks old? Even though they've not actually required heat for five or six weeks now, at least, they do need to be gradually acclimatized to colder temps before moving outside.
Turning off their heat was the first step. Next, make their room a bit cooler each night by blocking the heat vent to their room, and finally, cracking a window to make it quite cool at night right before you wish to move them into the coop.
In addition, start taking them outside for increasingly longer romps on nice days. This is what's called "hardening" against the cold. By doing this, after about a week, they will be able to move outside with maybe just a little heat at night for the first few nights.