She can't fire you for going to the police, that's not a valid ground. It's harder for you though because you're simply volunteering you're not an employee on the payroll so it would be easier for her to get rid of you as you can't really involve an employment tribunal.
Have you told your parents?
You have 2 months left, so just stick it out I think unless you feel really uncomfortable. Maybe try and gross her out, don't wear jeans wear loose fitted clothing and just try and keep away from her. It appears to be bothering you a lot though so you should definitely tell your parents right now anyway. Also is this a big charity, maybe formally complain to their head office in writing, you should send an email quoting what your manager said and what 'Jenny' did.