The first thing to do is figure out how much sun you will have in the garden. You could start observing that a few times each day with your kids! My garden has partial shade in winter, and full sun in summer, since the summer sun is directly overhead. So, if your shade is coming from buildings or trees on the south side of your area, you may get sun this summer!
I have a fun little book called "Roots, Shoots, Buckets, and Boots" about kid-friendly garden activities. In it, the author, Sharon Lovejoy, starts with her top 20 plants for kids. These are: pumpkins, sunflowers, gourds, corn, berries, hollyhocks, carrots, mimosa, poppies, tomatoes, trees, alliums, potatoes, lamb's ear, four-o'clocks, evening primroses, radishes, nasturtium, moon plant, and lemon verbena.
In our garden, my kids' favorite has been peas. The whole plant is edible, so they eat the little pea shoots in the beginning, then the actual peas later. (You sow pea seeds thickly below a trellis, then they climb straight up.) They also LOVE sungold tomatoes. One plant will grow 5' tall and yield hundreds of delicious orange-yellow small tomatoes. Both of these plants are easy to grow! For a beginner, I'd buy pea seeds and look for a sungold plant to transplant at the farmer's market or similar. Another thing you could plant now in a container would be any sprouting potato (preferably organic from your co-op). I also like mint, because you can add it to tea and lemonade, and it smells awesome when touched. Here, it's a little invasive, so I plant it in it's own spot (encircled by gravel), or in a container.
I think if the spot is beautiful and has something edible in it, your kids will find a way to be interested! Good luck!
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