Once you've decided what to grow, the next decision is where. Whether starting outdoor plants indoors or planting a windowsill garden, being creative is half the fun. Yogurt containers are good choices, because the lids serve as saucers. Pint sized ice cream containers and butter tubs work well for the same reason. In either case you need to punch drainage holes in the bottom before you begin. But don't stop there. Egg shells broken around their 'equator' and replaced in a styrofoam egg carton are a wonderful way to begin small seeds. When ready to transplant, the shell need not be removed. Plastic milk cartons, cut around their middles (use the bottoms, but save the tops) work well also. One young man I know started his seedlings in the bed of his plastic dump truck. Remember that the idea is to have fun.
Choosing a Garden Site
Assuming that at least some of your plants are eventually going to make it off of the windowsill and out of the house, where are they going to go? Again, a little creativity helps. Ideally a child's site should have good drainage and lots of sunshine. It should also be only about as wide as the child is tall. Any larger and it becomes too difficult for the child to manage.
I've found that an old, tiny, inflatable swimming pool helps here. Filling it with starter soil gives you a controlled, pristine environment. It's free from rocks, weeds and other nuisances. Or try an old wash tub, or a circle of old bricks filled with soil. When gardening, avoid using anything with petroleum or other chemical contaminants, treated wood, and old tires.
Finally you're ready to plant. First mix a little bone meal into the soil to encourage root development. Then plant according to directions. Then have your little one tear newspapers into strips and lay the strips, four thickness' together, on any spaces that don't have seeds in them. Keep everything, including the strips, properly watered. This is a low maintenance way to inhibit the growth of weeds.
In case of bad weather, use the tops of the milk containers (or the tops of soda bottles with caps removed) as miniature green houses to protect your delicate seedlings.
If all goes well, your little one will be enjoying the fruits of her labor in a short time. It's definitely worth the effort to see his delight when that first little seedling breaks the ground. And another gardener is born. Happy planting.