I don't worry about it, I have insurance and make sure my home is safe. Accidents happen and I try not to worry about things beyond my control. I always have parents sign an accident report if anything does happen.
I have had some pretty good accidents with the kids in my care, just last week my 4 year old dcb did a belly flop off the top of a snow hill and landed on his face. He was all scraped up and it looked aweful, but it was an accident and it isn't going to keep me from letting the kids enjoy the best parts of winter play. I just make sure the safety rules are clear for the kids to make sure it doesn't happen again (this dcb loves to fall down on purpose).