A.S.
If it were my child I would definitely want someone to tell me, even if it hurts. This girl sounds like she has a real problem and needs some help! If you think her parents will overreact, maybe you could tell her school counselor or something? You may not be the only victim of her sticky fingers and it's better that you call her out than the local police officer at the drug store, right? I think you explained yourself well and have lots of incidents to cite, so you can say that you didn't want to say anything at first and give her a chance to change her ways, but that has not worked. You sound like a caring person, so I'm sure you'll find a way to do this in a diplomatic way. I think you should see how this conversation goes before deciding whether or not to babysit her next summer.
Good Luck!