There should be a "lovable quality about the actual tools" you use in the garden, Gertrude Jekyll believed, and it's a rule that also should apply to child-sized equipment.
A tool that feels good in the hand—well made, properly weighted, just the right size—may inspire affection for the avocation of gardening as well. Here is a selection of child-sized tools from British manufacturer Burgon & Ball:
Above: Tempered stainless steel with wood handles, a Children's Hand Trowel (L) and a Children's Hand Fork (R) are £7.95 apiece from Burgon & Ball. For US gardeners, the tools are $17.50 apiece from The Arbor.
Above: The Children's Hand Fork is a comfortable size to maneuver in a container garden. Photograph by Alexander Fleming, via Flickr.
Above: A Children's Digging Fork (L) is 19.95 from Burgon & Ball. It's a little more than 30 inches long and made from ash wood with a riveted socket. The hand fork (R) is about eight and a half inches long and has an ergonomic, long-angled neck.
(N.B.: For more pint-sized tools, see "A Children's Garden of Verses.")