Imagine yourself trying to ride a bike when you can't reach the pedals, the brakes or sit on the seat and reach the handlebars at the same time. That's what it must feel like to a child when they use adult-sized tools to garden. The tools are too heavy and big for their little arms, hands and legs. Provide real child-sized gardening tools not plastic toys.
Get small-sized lightweight hand shovels, trowels, cultivators and weed pullers. The weed puller looks like a flattened screw driver with a "V" cut into the end. Look for smaller tools that have 30-inch handles and are lightweight. Child-sized rakes, shovels and hoes are available. An alternative is to convert an adult-sized tool by replacing the handle with a 30-inch dowel. Sand the dowel to get rid of splinters or wrap with tape.
Watering cans come in various sizes, colors and materials. A pint-size can is the right size for a toddler. Preschoolers can manage a quart-size can. Use a plastic pitcher in a pinch. Allowing the child to handle the garden hose is fun. Be warned the plants might not be the only thing that gets wet. Set the nozzle on a gentle spray so seedlings don't get washed out by a strong blast of water.
Carry and Tote
Find a plastic carrier for household cleaning and use it for the garden tools. They have compartments for different size bottles, clothes and tools. The size isn't too big for a child. Some big box stores carry a child-size garden tote. It looks like a bucket that has an apron with pockets around the outside. Tools are placed in the pockets and the bucket handle allows it to be carried.
An exception to the no-plastic rule might be a wheelbarrow. Plastic is lightweight and easier for a child to push. Make sure the wheel is sturdy. A wagon is an alternative to the wheelbarrow. It's a little more difficult to handle in garden dirt but easier to find in stores.
Besides sunscreen, a wide-brimmed hat keeps the sun off the child's face. Spray the inside of the brim with insect repellent if gnats or other bugs are a problem. Garden gloves are a good idea as well. Sunglasses provide eye protection from the sun and from stray dirt flying up while digging. An inexpensive pair of boots keeps mud out of the house. An alternative is to provide a pair of shoes that are only worn in the garden. A piece of rubber matting is good for sitting on in the dirt and protects tender knees when used to kneel upon.