Oh my Gosh woman! I am 31 weeks pregnant and barely take all my toddlers for walks anymore at this point. There is so much black ice hidden, even on the milder sunny days. I know for me personally, my husband would be flipping out if he knew I was walking as much as you do and would absolutely not be okay with me putting a baby in a backpack carrier with huge potential for falling.
I think you should: talk to the school agers parents, mention how the weather is just too risky for you right now to do these pickups, also look into another student doing drop offs and pick ups.
This could be a LONG winter that may last until March. Really think about your safety. It would be much worse to fall and have immediate contractions/early labour than to stop doing school pick ups and drop offs IMO.