I think I will insist on showers every other evening and add some pantyliners to the sanitary towels and re explain again how discharge etc is natural.
And you aren't her boyfriend either.
At what age did you start having fantasies and what prompted them? It was totally out of the boxers and she was staring at it.
If you think you can ignore her and wait it out till she realizes the age gap and it won't make sense to try anything.
The idea of getting her to help with her own washing is a good one too.
Oh and by the way, you bought an extra pack of liners for her as she is growing etc.