I know you are collecting rocks for your hardscapes but I saw this product at Menard's (a store here in WI just like Home Depot but cheaper) that has something called a Germination Blanket that works great for use on hills and ravines. It retains water and helps seed germination. It is also biodegradable and it covers up to 50 sq. ft. and costs $12.99. It is a little pricey for a large space but maybe if you have a walk-way for the girls or something. This would be so easy to grow some grass with your clay soil.....just put the seed on the blanket and water and you will have grass. If I could shake your hand right now I would bet you $100 you could grow grass with this system. The girls could even walk on it and their are no chemicals! I think I'm sounding like a salesman for this product but I have worked with something similar in the landscape trade and it truly works. It was typically used on very steep hills to help with erosion control.