This is the real purpose of the children's garden, to build warm memories of special family projects. Don't let it become a frustrating, stressful chore . Never tell kids something you could show them.
Young kids have a very short attention span.
Make sure that you have lots of options available so they can get started immediately and stay busy. Digging holes is one thing that seems to hold endless fascination. Instant gratification helps a lot. When giving out supplies to several kids, try to keep seeds, tools, etc. as similar as possible to avoid the inevitable squabbles. After an activity, do something to reinforce what everyone has learned.