Ground Cover Other Than Grass

I have a septic bed that is going into a large childrens play area. I would like to keep it a play area, but not put grass in, or clover or periwinkle that is too high. I have some strange low-weed type ground cover in the front that spreads and has tiny yellow flowers. It is great, but I would need a lot of seed to cover my bed. Do you have some suggestions?

My three favorites are: Rupturewort, Herniaria glabra (Zone5-10), Veronica filiformis, Creeping speedwell (Zone 4-8), and purple carpet lemon thyme, Thymus praecox articus 'Purple Carpet'(Zone 2-9).