Have the child make a special ornament for his/her garden.
Pick seeds that are easy to handle and plants that are easy and reliable such as Nasturtiums, Marigolds, Zinneas, Scarlet Runner Beans, Sunflowers, etc.
Plant the children's garden over several days, if necessary to maintain attention.
Start seeds inside under florescent light for slow growing flowers, especially in northern areas. Kids enjoy having things growning.
Plant the child's name using alfalfa seed or grass seed planted in a tray of cotton fluff or potting soil
Have child decorate a styrofoam cup as a face. Fill the cup with dirt and plant it with grass seed. As the grass grow the child can give it "haircuts".
Make stepping stones for the children's garden that feature their hand &/or foot prints from concrete (add fiberglass filaments to strengthen for cold weather durability).
Grow a carrot tops by cutting off all but 1" of the carrot(potatos, sweet potatos, other root vegetables work as well). Stick 4 toothpicks into carrot around sides and place over a cup of water with the carrot slightly into the water. Plant orange seeds to start an orange (grapefruit or lemon) tree for in your home.
Save seeds of flowers (marigolds, sunflowers, etc)or vegetables (beans, pumpkins, etc.) to plant next year. Place seeds on paper plates until dry and store in a sealed jar until planting season. It is good to learn that we don't have to buy seeds, we can find them and save them.
Take cuttings together from ivies, geraniums, or other easy to root plants. Plant a terrarium together in an old aquarium or large glass bowl.
In the garden, children like extremes. Plant huge sunflowers and other large plants in one area. In another area grow tiny plants-maybe make a "fairy garden"!