T.K.
K.,
Are you trying to cover asphalt or concrete? If not, just let them fall on the grass, dirt, sand, whatever is in the yard now. My 3 boys have lived in several different backyard type environments. One was a completely concrete backyard with a landscaped border...there was no way to cover that so they didn't get hurt. They survived. They have lived in places with lush, soft, grass, they fell from their swings and survived. They have lived with a very rocky back yard, they took some spills and survived. They have fallen on playgrounds and play areas that have mulch, deep sand, or that rubberized stuff, and they have gotten scrapes and and bumps and they have survived.
I guess what I am trying to say is there is no real way to keep your kids from getting hurt when they are playing outside - other than just not letting them play. It can be scary if you start thinking of all the risks and all of the "what ifs" when it comes to kids. If I think back to the crazy things I did and the spills I took as a child I don't know how my mother didn't have a heart attack.
Good Luck,
T.
(mom of 3 boys - 9,7,3)