First of all, sorry for the lack of a "real" picture. My computer is down and out with a virus (getting fixed) and I am without photos, so we will rely on Google's ability to scan the Internet.
Gardening has got to be one of the most educational activities you can consistently do with your child. Growing a garden provides such a learning experience, from prepping soil, to planting, to watering, to watching little seedlings pop up, to watching them grow bigger and bigger, to harvesting and eating the food--not the mention the "interesting" bugs all over.
We grow a rather large garden and I have my children help with each step. They help plant seeds. Do the drop seeds and sometimes plant too many? Yes! Especially with things like lettuce seeds which are rather small.
I also have them help me weed on almost a daily basis. This is when the most learning happens. They ask question after question about what we see and do. They learn to identify a plant by how it looks. They can tell the difference between a plant and a weed. They watch the fruit and vegetables slowly grow on the plants. They ask about bugs. We talk about sun and water and other things important to a garden. Children are so incredibly observant--they will likely even point out trends in your garden you aren't picking up on.
If you have a garden, involve your children! If not, consider growing something (Manda has talked in the past about small garden growing) next year. I can promise you that you will be absolutely amazed at the things your child will learn.
What ages can do this? I would say most two year olds and older can help. When Kaitlyn was two, she had the hardest time figuring out where not to step until plants popped up. I had to take a deep breath, tell her again, and remember she was two and also far more important than any seed in the dirt :). When Brayden was two, he totally got it and was fine. So you never know. But once the plants were up, she was fine.
You will have to watch a two year old much more closely than an older child. But even just one year later at 3, Kaitlyn was perfect in the garden, and she is recognizing plants.
McKenna is a pre-toddler and she joins us in the garden. She digs in the dirt in designated areas and she will help harvest food by loading it in the big bowl. She is 16 months old so filling containers is right up her alley right now.
Another great bonus of the vegetable garden is that you will find your children eager and excited to eat the food they have watched grow, even if it is a food they usually wouldn't eat. I could go on and on. I love the garden! (notice I said THE garden and not TO garden--ha!).