Encouraging kids to take part in the healthy pursuit of gardening can only be good for them. Most kids are fascinated to watch a plant emerge and grow from a seed they've sown themselves.
Start them off with larger seeds or bulbs in pots, or develop a small veggie patch for them. My kids love to take charge of their own patches, each about 1.5m by 1.5m.
Teach them how to sow seeds and plant seedlings. They usually enjoy watering their plants and if you're really clever you may even get them to do their own weeding (but watch out for stinging nettles).
Ownership is important. Let them choose what to plant and make sure they do the planting themselves. Your role is to teach and encourage.
Also be well focused on safety. Teach them about care with tools and make sure your children get into the habit of wearing gloves while gardening and washing their hands after.