Why don't you plant runner beans too as they produce food? If you make wigwams with gardening canes the beans will grow up them. Plant the beans in pots and leave them indoors till they germinate, then write the planter's name on a lolly stick to put into the ground beside the plant when you plant it out, and have a bean race. You could also make fruit salad and extract pips and seeds to plant. Particularly spectacular is planting pepper seeds. Cut open a red pepper while the children are watching, plant the seeds and care for them. You will get green peppers that will grow and 'Magically' turn red. Set beer traps for slugs and snails and collect ladybirds to eat aphids.