Preschooler:
This is an activity that takes time and patience, good things to start to learn at this age.
Take your child outside with a small paper cup. Fill the cup with dirt and plant a few small
seeds. Explain that seeds need three things to grow; dirt, water, and sun. It is a good time of year to start your seed because you will be able to transfer it outdoors into a larger container or garden in a few weeks.
Then put the cup in a window sill. This will take some explanation, especially if they have seen a seed grow in a few seconds on TV. (like my son had) The concept of time and waiting is developing in your child and they will need help to understand why it didn't grow "right now".
We used sunflower seeds and had small buds in a few days. We also needed to discuss how much water a small seed needs. :-)