I have 2x1 yr olds, 3x2 yr olds and a 3yr. They don't enjoy the outdoor time if their drool/runny noses are freezing to their face. We go outside to play if the temp including windchill is -15 or warmer. We have a large playroom with trampoline and ride on toys and built a fort on the days it is too cold to go out.
I have 3 kids of own, boy and girls so I have a huge supply of back up weather gear. If one of the kids forgets something, I can almost always supply it. I have never stayed indoors because of forgotten items due to this reason.