The problem with using a tree to try to teach these lessons is trees grow too slowly. I agree they don't teach exactly the same thing, but the end goal is to make kids aware that we need to be aware that our lives are intertwined with the health of the earth. Seeing the growth cycle at a level that kids can comprehend will do more for that than planting something that, once planted, effectively becomes little more than a piece of scenery to them.

The importance of water conservation, how we interact and effect the water cycle, air quality, and many other things can be taught more effectively using the much-faster growing vegetable garden, and, since the kids have a vested interest (they get to eat what they grow), they are more engaged in these lessons.

I'm not saying kids planting trees is a bad idea, I just don't think it would be a very useful teaching tool for internalizing the importance of conservation.