"Tree hugger", as a pejorative, typically implies a person who appears to have lost a sense of balance in the environmental discussion. People aren't going to move back intro thatched roofed, dirt floored huts, and, when used as a pejorative, that is the inference being made. It's a case of using extremists to invalidate large groups of people. Typical human.
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 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.
Empress trees grow 10-15 feet in a year.
A weeping willow will grow about 5.5 feet in a year.