Elduderino - you must be a good man! Thrifty, practical, handy, and teaching your kids to ski!
I have done the same thing many times - buy some skis at a swap for the kids and try to make them look like new. All good ideas above, just depends on the severity of the abuse. For badly scratched up top sheets I have used 600 grit sandpaper, then 1000 grit, then hit them with a couple light coats of clear coat - with truly amazing results! Is the clear coat as durable as the factory finish? No, but it will make a great first impression and they will hold up fairly well for a season. And then they are grown out of them anyway, right?
Go to a paint store and tell them what you are doing - ask them to steer you towards a paint that is more durable and flexible instead of hard and brittle. That way it won't be as prone to chipping/cracking.
Good luck and put a smile on their face!