All I have is my own personal experience to speak from.
When I was in 2nd grade, I spent a winter in Maine while my father was in a 3 month long training program, so we were temporarily living with my grandmother. It's not a news flash that it snows a good bit in Maine in the winter, and there was snow on the ground a good portion of time I was there.
I specifically remember that about 5 minutes before every recess, people that brought "snow pants" were allowed to go get them and put them over their regular pants (or dress, I suppose for girls). I remember putting them on myself and taking them off afterwards. Took me a good while to get the hang of it, but eventually I looked like the rest of the kids doing it. I don't recall any teacher helping us.
Obviously there are differences all over the world, but it does seem that it is well within the abilities of the average second grader to take off and put on their own clothing.