Ask what seeds need to grow. (sun, water, soil). Explain that seeds need soil to grow because of the nutrients (vitamins).
Cut onion in half to show "food supply" - explain that bulbs do not need soil because they carry their own food supply. Have children help fill containers with Legos, or small rocks.
Plant flower bulbs (pointed side up), in legos (proves bulbs do not need soil)
......bulbs will thrive if watered and given sunlight - eventually will flower inside.