I won't pretend to know how to parent, but out of curiosity, have you communicated this to her? It sounds like a good lesson in teaching people to communicate and come to mutual understanding and resolve it together through communicating an agreement rather than an exercise in conformity based on commands from other people's desires.

Again, I'm just a nerd on the internet and I don't know what I'm talking about, but if you do try that, let me know what happens XD