Your first priority in getting your kids involved with your garden is to make the work fun with bright colors, lightweight tools (but not plastic) and a sense of whimsy. Start with what they like to do most. For instance, a child-sized shovel and a small wheelbarrow make digging and carrying things around extra special. Add other tools, one at a time, as you ask them to help with other chores.
Start with a folding shovel sold for camping, and move up to larger sizes gradually. For older children, choose small shovels for women, sold in the the main tool section of many hardware stores. Also consider tools sold for the disabled. Many have handles that extend and smaller blades.
Durable, bright-colored wheelbarrows are sold in many sizes, from tiny ones suitable for toddlers to carry toys and seeds in---to small adult garden carts, just right for kids seven to ten years old. Wheelbarrows have one wheel and two supports at the back, while garden carts have two wheels at the front and two supports, features that make them more stable. Either style works for children, but make sure there are no sharp edges.
Essentially shovels small enough to use with one hand, trowels are useful for digging holes to put small plants in. Look for very lightweight shovels so your child's arms don't tire. If you can find one in a bright, cheerful color, so much the better.
A small bamboo rake is easy for a child to use, even if the handle is a bit long. Also, telescoping rakes are useful for adult chores such as getting leaves out from between perennials.
Children love watering plants and appreciate having a watering can that's theirs exclusively. With the many choices of colors, decorations and sizes available, you might want to let your child pick this one out. Just make sure it's not so large that carrying it across the yard is tiring.
No matter how much fun it is to get their hands in the dirt, gloves are useful additions to your child's garden kit. Tools can abrade soft skin and, besides, the bright colors now available add to the fun.
When your child is old enough to pick flowers under your direction, a pair of small round-nosed garden shears proves useful. Look in the adult garden tool section of your hardware store.