I don't want to throw a spanner in the works, but are you sure your 'volunteer' is the only culprit? Could anyone else get to the money? Did she know you were actually charging?
It would be awful if this person was innocent especially as she is a client as well. I think I would say something to her directly . It would be awful if it came from someone else and you know how these school mums like to gossip .
Plus, it will play on your mind forever and if you're anything like me, I would just blurt it out one day and upset everyone involved. I think it's better to get to the bottom of it now, don't let it brew. If it is her, she may well not admit to it, but she'll have to live with the guilt.
Nobody could get to money other than me, she knew the prices as she was told and they were listed.