Our town's parent group sponsors a ski exchange. It's for equipment primarily but it would include goggles and mittens and ski pants too. Kids outgrow stuff before they wear it out. Look around and see if there's anything similar. I'm not sure why you say second hand is "out of the question" - is that because you have no resale shops near you, or because your family won't consider it? Also, around here, resale shops often have new items (with tags still attached) because someone bought something or received something as a gift that they never used.
If you are putting 4-6 year olds in full day ski school, be prepared that they need to get out of all those clothes periodically to go to the bathroom as well as eat lunch, so the clothes need to be easy to remove and well labeled. So you need some "spare parts" because kids will lose their goggles & mittens, and the instructors don't know the kids well enough to keep track of their stuff.
Layers are usually the best way to go to prepare for a variety of weather conditions. If weather is really foul, I doubt that kids that age are going to be so amenable to being on the slopes for long periods of time. Can you find out if they have dryers available during the lunch break?
I wouldn't break the bank on this until you know for sure that your kid is really going to enjoy this sport as much as you hope. It's an expensive sport, as you know, with lessons and lift tickets and resort/mountain fees, even before you add in the skis and the clothing.