My girls have helped me with string beans, peppers and tomato plants. They have also planted flower seeds like sunflowers. This year we did snap peas, we started them from seeds and my youngest planted all the seeds. They are starting to come up and she can't wait until they grow enough to eat them.
I find that when kids help you grow a veggie plant they are more likely to eat the vegetable because they planted it or helped to take care of it.