Are the youngest being allowed up on the roosts at night? Does the coop appear super dusty, with dust visably floating about? Every coop will have dust, but other than cleaning day, you shouldn't see it floating about. The older hens are allowing the youngers to eat and drink freely? Just because there is food/water out doesn't mean the youngers have access to it if there is a bully in the older group who is keeping them from it. I'm not saying the cold isn't affecting your youngsters, because I don't think young ones take it as well, but I think there are other factors involved based on your description of they eye issues.
If your coop has ample space, you could try a heating device to see whether it makes a difference; just take every precaution you can. Never just clamp a lamp; hang it securely from a small chain or heavy wire to two different points (one as a back-up). Be sure to get a lamp with a guard and a ceramic base. Make sure your base is rated for your bulb. If your base is rated at 150w, use a smaller bulb, maybe 100w. Turn the lamp off each morning. Make sure there is plenty of clearance between your lamp and anything that could burn (including your chickens' feathers). If you're running an extension cord, make sure it's rated for outdoor use.