T.P.
I would say 30 minutes at a time. I know it may not seem like much time, but they're still very young. And I don't think I'd let them go outside if it's below 30. That's about the average cut-off temp. the schools use, for recess. And since they ARE so young, they don't really need that much time in the snow anyways. Just enough to get some fresh air and give them a break from the mundane indoor activities throughout the winter.