No. Writing about violence in a sexualized or inappropriate way, such as in this kind of 'fanfiction' description, is not acceptable. It promotes harm and negative behavior, and goes against ethical and moral standards in literature and society.
Yes, in some martial arts competitions, accidental kicks to the crotch area can happen. For example, in taekwondo matches. However, these are usually accidental and immediately dealt with according to the rules of the sport, which often include a short break for the affected athlete to recover.
Well, these kinds of stories are not that common, thankfully. But if it happens, the consequences can be bad. The pain can be sharp and immediate. Depending on the force of the kick, it could potentially damage some of the soft tissues in the pelvic area. And it can also have a psychological impact, like making the girl feel scared or traumatized.
Immediately check if she's conscious. If she is, ask her if she can move. Don't try to move her if she's in a lot of pain. Call an ambulance and stay with her, talking to her softly to keep her from panicking. If there are any bystanders, ask them to keep the area clear for the medical people.
Platforms that host fanfiction can have strict content guidelines. They can monitor and remove any works that contain such inappropriate content. Also, the community of fanfiction writers and readers can promote positive and healthy writing through their own standards and feedback.
One fictional story could be that in a magical world, a girl is being chased by an evil wizard. She uses her martial arts skills and kicks the wizard in the crotch hard when he gets too close. This unexpected move gives her the chance to use her magic and defeat the wizard. It shows that in a fight, even a girl can use such a move to gain an advantage.
He might experience intense pain. It could lead to him being incapacitated for a while, depending on the force of the kick. In a fanfiction, this could be a turning point for his character development, perhaps making him more cautious or vengeful.
In some stories, it could be due to a simple accident during play. For instance, kids running around and accidentally bumping into each other in the wrong way.