Be an example
I beleive practicing is better than preaching and as someone has said, children may not always listen to you but they observe you all the time. Therefore if you are a fine example of an environmentally concious person, your children will automatically learn to love nature. I have learned so myself seeing my parents planting trees, going for nature walks etc. and now see my son following my example and watering the plants, talking to them, like he sees me doing.