Water
Following the instructions that come with the bulb, plant a daffodil bulb. Care for the bulb by feeding it regularly with water.
Use diagrams and observation notes to keep track of its growth and changes throughout the year (this can be adapted according to age - a simple drawing for younger children, observation notes for older children).
You could also try planting some daffodil bulbs in different places (full shade, half shade, and full sun) to compare the growing conditions and results.