Planting trees and planting a garden call to disparate lessons!
Planting trees is about ecosystem conservation.
Planting and tending a garden is about understanding plant life cycle, the nitrogen cycle, /and/ where food comes from.
Unless the gardens you're thinking of planting self-sustain their nutrients, conserve (or even generate a surplus of) water, and have zero impact on surrounding ecosystems (fertilizer and pesticide runoff), they don't really teach anything about ecosystems other than how we can manipulate them for our gain (greedy or otherwise).
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.
I would suggest that planting trees is part of planting a garden. Trees are a renewable resource that if, managed well, produce so many benefits. Where I live, 100's of thousands of acres are managed over offset 30 year cycles.
Planting trees is about ecosystem conservation.
Planting and tending a garden is about understanding plant life cycle, the nitrogen cycle, /and/ where food comes from.
Unless the gardens you're thinking of planting self-sustain their nutrients, conserve (or even generate a surplus of) water, and have zero impact on surrounding ecosystems (fertilizer and pesticide runoff), they don't really teach anything about ecosystems other than how we can manipulate them for our gain (greedy or otherwise).