I have a newsletter and a dc fb page that I put reminders in/on. This was the first year in 4 winters that I got more specific ( toques must cover ears, mitts must cover wrists, neck warmer's instead if scarves etc) because I find it much more enjoyable for both them and I if they are in appropriate clothing. I also stated that it's not necessary but I prefer one piece outfits. In the past I've had parents leave a one piece here and just use a jacket for going between car and homes and it's the best! Makes getting little ones dressed so much quicker! Not all parents can afford to have 2 outfits though and some just used the one piece all the time. It also REALLY helps with the dreaded forgotten ski pants if they take them home!! This year I've bought a one piece as a backup because I got it super cheap, used.
Some parents can't handle getting their child dressed though so those ones tend to use 2 pieces because they only have to put their arms in the jacket and don't bother with the ski pants lol. I had one mom who would use leg warmers because she couldn't get her child to put the ski pants on....however that didn't work with me and the child knew the ski pants had to be worn at dc!
If it's still one or two parents who aren't sending appropriate clothing than I'd talk to them directly and find out the scoop!