Hi Melissa, as a long-time girl scout troop leader, I've tried a couple variations, cardboard box/foil, and sunshade/large pyrex/small black pot. Both work, though I can't really say which is better because the amount of sun really does matter in cooking time. You also need to be patient (plan ahead and be flexible in when you want to eat!). We also didn't really mind if our corn bread was over-done on one side and not-quite-done on the other!
Our backyard has plenty of wood for fuel, so we cook outside a couple times a week with our family. The solar ovens I've made were just gimmicks and took a lot of effort for the result. Great if you're planning to have fun and an educational experience for your child, but I would only build one with "found" materials. Otherwise, a regular firepit makes things MUCH easier! You could also take a #10 can and poke some holes to make a small cooker (uses less fuel). Search for ROCKET STOVE and you can find a few plans...