That's a fun experiment and it's great to see your little ones involved. Reminds me of Sepp Holzer and his unique use of rocks to form microclimates. No matter what you grow or where you're growing it, keeping your soil covered is always a great idea. I love to use wood chips that are partially composted. It keeps the moisture in, protects the soil, and even provides continous fertilizer for plants. I may have to try mixing rocks in with the wood chips next year. Rocks store heat which is very helpful in colder areas, plus microbes in the soil slowly release minerals that were locked in the rocks. Keep up the great work.