With the tarp at night and for any nasty daytime weather (heavy wind or rain), I would expect them to be fine with a heat lamp in there, and pretty soon they won't need the heat lamp either (watch where they sleep: when they are sleeping away from the heat, they do not need it any more.)
I do...
Can the tractor provide reasonable protection from wind and rain? If yes, I would move them out there. Just make sure the ground isn't wet when you put them there.
It probably would be a good idea to provide a heat lamp for them in the tractor, at least at first.