There are a couple basic ways parents can help their children really understand and embrace environmentally-friendly practices.
Teach by example. Children learn many things, from speech to ethics, by watching those they love and respect. Doing your best to live as eco-conscious as you can will show them the right things to do without any effort. Kids emulate their parents without even thinking about it. Being kind to the environment will just be a part of everyday life.
Talk about green living in their terms. Kids minds are loaded with questions, and you never know when a teachable moment can present itself. Simply explaining why you want them to color on both sides of their papers or use up their crayons until they can't be used anymore helps them to understand why you ask them to do the things you do. If they know that there's a method to the madness, and that you're helping them to help the world around them by conserving, the lesson is more likely to be learned and embraced.
Besides setting a good example and talking about living green, giving your children materials that help to explain things and reinforce what your teaching them helps, too. Pick up a book, like Ian James Corlett's E is for Environment, that supports green living through fun, relatable stories they'll enjoy and learn from.
Teaching your children the little efforts we all make add up to a large impact is easy and virtually effortless if you start early. Living an eco-friendly lifestyle will be a natural part of their everyday lives, and they won't find themselves trying to adjust their habits to live greener when their adults.