Watermelon can be a little difficult. Carrots are pretty easy to grow, as are strawberries, as long as they don't get dried out. Bok choy grows fast in the spring, but doesn't do well in the heat of summer. Broccoli is also good for spring. Green beans are super easy and produce a lot if you get the plain green(Kentucky wonder) type. I tried the colored ones, but they didn't produce many beans.

Here is a link that might be useful: My Square Foot Garden

snow peas for early spring, really prolific and hard to mess up and you can plant them pretty densley-they won't last all summer but when its time to pull them there will be plenty of time to plant another crop of something else, preferably seedlings started indoors.

Lettuces are easy to grow, come in many shapes and colors, grow very well in a container and you can start picking small leaves in just 3-4 weeks. Cucumbers and green beans are fairly easy to grow if you have a frame for them to grow up.

I think that the most important part is that your 11 year old like eating what ever they grow, if they wont go near a radish it will not matter that it is very easy to grow.

Good Gardening, Mary