I don't think Casey attempted to bury Caylee in the backyard, nor did Cindy or George. The cadaver dogs "hit" on a couple of spots... which means a simple alert. If their had been a body there, they would have indicated a very strong alert, and LE would have dug the whole yard up. It was likely something which had come in contact with the decomposing body long enough to absorb some of the odor was placed on the ground, like a blanket, article of clothes, the shovel, or the gas cans.
Of course, it is possible that the body was placed there for a short time and moved later, but I don't think it is still there. I could be wrong, but this is just my thoughts from reading information about cadaver dogs here.
Oh, I think madam needed a shovel for a reason. She's not an industrious person. She wouldn't have been out "working" in the yard to help her parents, for heavens sake. She probably needed to move something, or possible evidence connected with something. Which explains the hits....