What about starting with some easy things and start investing in the more expensive trees and bushes after some success with herbs and veggies?

We have had wonderful luck with peas and bush beans. The kids will eat them off of the plant. They also loved carrots.

Herbs are very decorative and your kids can pick them for use when making meals.
Chives
Oregano

You could also plant strawberries, I think they make lovely landscaping if they are controlled. We grew some from 25 starts and got some berries this year (the kids were excited about it), the plants look really great now and next year they should produce even better.

how did you plant the strawberries? the rabbits got all of mine....