Really no temperature is too cold for kids to be outside as long as they have coats on and are bundled up really good including wearing scarfs, stocking hats etc.
When I was a kid I played outside for a couple hours in below freezing temperatures and I'm still alive today.
Just because it's below freezing it doesn't mean you or your kids have to stay inside the house.
Go ahead and let the kids outside to play for at least an hour at a time as long as they're bundled up warmly and they come inside at least after every hour for 30 minutes to warm up they'll be fine.
I have a friend that makes her kids stay inside the house nearly all winter long except to go to school and her kids hate that.
Then she's glad spring comes around so the kids can get outside the house. lol
Hello kids can go outside in the cold and even adults can for awhile.
Just because it's very cold outside it doesn't mean you can't be outside in the cold.