there are a variety of variables at play...
1) are they fully feathered out?
2) what are your overnight lows?
3) you say they are still on a heat lamp inside -what temp are you keeping them at?
4) how warm can you make it outside with the heat lamp after it cools off considerably at night?
in almost any case, you can almost definitely start transitioning them now. get them outside in a run during the day for ever increasing amounts of time. as long as they have protection available from any sun that may be shining or rain that is falling, they will be very happy. a bit before dusk, turn the light on in the coop and either they will be attracted to it and go in on their own or you will place them in there until they get the hang of it.
this will serve a couple purposes. it will get them out of your bathroom for at least part of the day. it will make them happy and healthy. fresh air, exercise, more to do, etc. it gets them on a more natural light type schedule. it will make the eventual night spent outside a bit less shocking, though if you put a lamp in their coop and they aren't cold, they will be totally fine. it's when you turn the light out that there will be protests. but that time comes for every little peep. good luck
