I agree with what others are saying. Once you accept money you aren't acting as a friend or anything, but a professional.
They aren't just paying for your time to teach their child, but also their daughter's safety. This is further complicated by it being your horse.
I'm not sure how well a release would hold up, most of the ones I have read tend to cover accidents rather than "mis-instruction" which could end up being a potential problem.
Many people say "nothing will happen" but it can and does happen, and if I were you I would be cautious.