I did read in a health advisory form (a couple of years ago), that children should wear snow suits/pants/jackets when the temperature goes below 10 degrees.

In my policy documents and checklists I do ask the parents to leave outdoor play wear appropriate for the season at my daycare for just this sort of eventuality. Admittedly it is a pain to store it all The amount of times that a child has turned up minus a hat or mittens.....and I've only been running for 7 months....lol