I am also a little more lenient in the spring and fall "transition" seasons. However, if you think about it, mornings are always colder during this time and there is NO excuse for having your child in a flimsy little sweater that doesn't keep them warm.
I am constantly reminding, and putting notes in the newsletter. If it got really bad, I would absolutely send someone home in the morning to go get a warmer jacket or a pair of pants.
I simply do not have the space (or the desire, hahaha) to keep a spare everything around here. Since I have both boys and girls, I probably have something, but I make no promises.