I read through your short story, I suppose we could call it flash fiction, and felt there was a nice outline there. You had clear conflict with the daughter trying to murder the son, with a nice twist at the end. One of the questions I had was: what was the girls motive? Why does she want to cause this boy harm? What has he done to her, or caused to happen to her?
The reflections at the end would make excellent writing prompts. As a short piece of fiction, I felt that the plot was good but the characterisation was lacking - the daughter didn't feel real and we knew very little about the son. Granted, this is difficult to do in flash fiction, so my suggestion would be to redraft this story as a longer piece to allow you to fully explore the motives of the girl and build suspense - perhaps she could be thwarted in her attempts several times before getting hold of him alone?
Just my opinion. If you do decide to rewrite it, please do let me know so I can read it. You've got a great plot there and I think it would work well as an outline. To help add depth to the characters, consider the following things:
What are their motivations?
What are their strengths? Hobbies? Weaknesses?
How do their personalities compare? Who gets more attention from their parents? Why?
Have any events in their past shaped their personalities? For example, why is the girl so willing to hurt the boy initially? Has she had an experience that has caused to act in this way?
As I said, a great story, but please take my advice as just the subjective opinion of a fellow writer. Keep it up, the hardest thing to do in writing is create an intriguing plot and you've done that well.