I would have probably left them in the building. Made a warmer bed for them if it was really cold with the door open. Get a cardboard box and put an old towel or blanket in. I say mama cat was just out of eye site and waiting for the coast to be clear to return.
My wife found a very young kitten in some heavy machinery where she works. We thought it was fine seeing as it would come to you wanting attention, purr and eat. We took it to the vet the next day and it had inoperable internal injuries so they had to put it down. That's the catch 22 with cute cuddly kittens/doggies. Sooner or later you're going to get your feels all shit on because well...it's just how life goes.