My oldest are 3 years old now and she loves to help me in the garden. She helps me to cut of the dying roses, and put all the leaves in a bag for drying. When we plant veg seedlings she has her own to plant and have to help with caring for them and weeding their patch. She can now show me what is weeds and what not. I let her identify the fruit and veg in the garden, and all the colors of the flowers as well. She helps me pick strawberries, tomatoes, beans, lettuce and a manner of other fruits and veg. I let her taste the herbs, and she helps me to take the flowers of the basil. In winter she helps raking the leaves, and carrying away the twigs when we prune the fruit trees. She helps watering the pot plants as well.
The big thing is to make it fun for them and not work, let them see the results of everything you do, and if they are older, give them their own little piece of garden where they can plant and care for what they want to plant there.