I think the first thing to consider is age. I think it's never too young. So long as they can pick something up and move around it's a good start. The best age would be at about 1-2 years old when they can start to hold things, walk and coordinate themselves properly.
You can buy plastic brooms and rakes which can help them get in and around the garden or outdoors to clean up any leaves or twigs.
The next step could be to have them pick any fruit or veg that you have growing in the garden. Something simple like tomatoes or strawberries is a good start where the colour of the fruit can stand out and it's not too heavy for them.
I think get them to handle fruits, vegetables, soils and even worms so they're comfortable with how it feels and what to look out for. This will help their confidence for when they get older.