Something to the effect that you will exercise proper precuations to prevent any accident or injury to the dog you are walking, but that sometimes unforseen things happen and you and you parents will not be held responsible for any vet bills or loss should injury or accident happen. Most of this time this would not be necessary but sometimes things happen and I have seen quite a few cases like this go to court and it can get expensive. I would just hate to see something like that happen when you are just trying to earn some extra spending money. Things didn't used to be that way, but anymore lots of people are sue happy and it's hard to tell who it will end up being unless something happened. It's just a precation to protect yourself and your parents but it's up to you.