Even at a young age, they realize that they've been trapped inside in a school for way too many hours. They're a little bit sensitive to the more pedantic lectures about environmental problems. An approach like that can actually have a negative effect. Some schools are getting kids outside, for instance monitoring their own water systems in a collaborative relationship with some research organization. The most powerful approach, the one that often affects long-term change, is when the children are actually asking questions of their own and investigating the questions they themselves find important.
Communication is the other thing.
When you teach for one, you don't always get the other. If you train for knowledge, for example, you don't often get changes in behavior. Environmental education programs have to have very specific outcome goals, especially with children, who need the abstract nature of things to be made more concrete. My main mission in life right now is promoting active participation and investigation. If education is going to be transformative, then it has to actively engage the learner in a way which is relevant to their lives.