You also have to take into account that if something were to happen to someone's dog while you had it in your possession, while walking it, and incurred either vet bills or death that you/your parents could end up being responsible for it. You may want to think about having a contract written up that must be signed by the owners in case something were to happen, that would absolve you of any responsibility as long as you took proper care and precautions to prevent an incident. You may think nothing could happen like that but it does, all the time.