When I was younger, I found it fascinating watching seeds grow into trees and plants. Why not get the kids to plant some seeds of their own choosing – you could even get them to pick some from fruits or vegetables they like to eat.
Avocado stones are quite fun to grow too – you can balance them with toothpicks over a cup of plastic water (the rounded base of the stone needs to be slightly dipping in the water) and you can see the roots come down very clearly. Make sure you change the water every day, and don’t place the stone in direct sunlight. When it looks like the picture on the left, you can plant it in a pot with some compost.