Eating what you grow yourself will be fun and educational for city kids. This can teach them to appreciate their food. Even if you do not have a garden or a balcony, you can still grow some simple plants near the window or along the HDB corridor.
The easiest and the fastest would be spring onions. This would be ideal for young and impatient kids. Just put small onions or shallots into a pot of soil with the pointed end poking out of the soil a little. Water it and after a few days, you will see green spring onion leaves that you can harvest for garnishing or soup. If you do have pots or soil, you can buy them from nurseries and markets. You can also try growing peanuts, though they will take longer to grow.
You can use the seeds you buy in the market. When you shell the raw peanuts, do not remove the papery skin as it is a protective layer. Peanut is nice to grow because you can see the leaves shut up when its night time and the way the flowers burrow into the ground to give you the peanuts.