Of course no one would care if it was $5, it was $100 which is a ridiculous amount for a child. It’s not about it being a physical note.
My kids are 6 and 8 and both believe in the tooth fairy and have a pretty solid understanding of what $100 could get them so I completely disagree with you on that.
It’s gross on a few levels. Posting it is completely in bad taste, people are struggling and everyone should have an awareness of that and be sensitive to the current climate. Her kid telling his friends at school could really confuse and upset the group and it also sends a terrible message to her son about the value of money.
Just because you can doesn’t mean you should.