We have been inside all this week - with windchill it's -31. Like others, I gauge it based on the youngest child. If all mine can run around and keep warm, we are more likely to go out on a cold day vs when I have little ones not walking and sitting still in the wagon. The cold gets to them so quite regardless of how well wrapped they are.

My cut off is around -20 with windchill but I will admit if it's -21 with zero wind, we are more likely to out than when the air temp is warmer but there's a biting wind. It's the wind that gets to them faster.

-10 would mean being stuck in for 2-4 months of the year.