I'm an esl teacher, does that count?
I always thought that the best way to reach kids is to show them.
Example: have a forest conservationist come and teach them about trees, go to a park, talk about plants, what they needs, what kills them etc.
Bringing kids into their own environment to teach (as oppose to a movie) and learning about how it affects them directly. You could also have a little project that they can do to win a prize (give them 2 weeks after or before your event todraw a picture or whatnot and you can take some of your budget money for an "environmental" prize...good for promotion later on too).
Good Luck
EDIT TO ADD: Billion tree campaigne would be awesome.