So basically you want to teach your son that it's a woman's job to do everything for him.
Also, this is because you have to buy her sanitary supplies? They are not that expensive and you probably don't actually even have to buy them every month. These are things she needs to be healthy, you should get them for her because you're her parent. They are not a luxury that has to be earned. The fact that you treat them like they are is sexist right there.
You're basically making your daughter apologize for something she has no control over.
I'm sure you've never had to buy anything special for your son at all...