There is no description, no explanation of why this girl “becomes” a boy. She is treated badly by her brothers - seemingly because she is a girl. She wants short hair. To go to the ball she dresses in trousers and a jacket and her mouse friends cut her hair. So, with short hair and trousers she decides she is actually a boy. As a boy she goes to the ball and effectively falls in love with the princess.
What message does this give children?
You like wearing trousers and having short hair?
You are a boy.
Men are unpleasant to you because you are a girl.
So.
Be a boy. It’s much better. You get to make a car out of a pumpkin, and escape the nasty men.
You fancy other girls?
You must be a girl.
Honestly, it is that simple in the book. It’s appalling. It’s obviously written for very young children. They will see it in a very literal way. There is NO WAY I’d give this to a young girl - or boy to read.