Your best bet would be a simple release of liability form signed by the charity. The charity would need to get one signed by the recipient, but they probably already have something in place to protect themselves from this sort of thing.
You can search "general release of liability form" and get a few ideas and see if you want to write one up yourself. Basically it is saying that they can't hold you responsible for any harm caused by the bow you are donating. Even with that, if someone gets seriously injured there's likely to be a lawsuit, but this would definitely help your case.
A lawyer could write one up in a few minutes.
Our family business deals with this from time to time being a small manufacturing shop and my wife is a lawyer. I'll ask her option tonight.