Our children were toddlers when we began bringing them to organic landscaping jobs. If we had been using chemicals, I would never have brought children along, because children who are exposed to garden chemicals have a greatly increased risk of developing leukemia.
So, children's gardening rule number one: avoid using chemicals!
Children's gardening rule number two: avoid poisonous plants! Learn to identify poisonous plants that are common in your area, do not buy poisonous plants, and if possible, remove the poisonous plants that are already in your yard. Teach your children that they should NEVER put any plant material in their mouths unless you give it to them!
Rule number three: NEVER use power equipment when a small child is in the yard. The deafening noise makes it impossible to hear what a child is doing, and even if the child stays far enough away to avoid being cut or run over, there is always the possibility that someone will be hit by a flying rock or stick.
Rule number four: give your children specific jobs to accomplish during the day. Our son's first job was pruning the dead tips off of a cedar hedge. We gave him a small, extremely dull pruner, showed him what to do, and let him do his job. It kept him busy most of the day, and made him feel important. He was about two years old.
Rule number five: children love wheelbarrow rides. Indulge them often.