I would just continue to let them out if they want to go out, but make sure they can go back inside if they want to. They will decide when its too cold. Mine love wading through the 2 foot snow and making duck sized tunnels
yea, I agree with cityfarmer12, they should be fine out as long as the can get back to their shelter. they will know what is too cold for them.
some of mine were out playing in the water this morning, and it was 20 degrees!