Rxrliu. What happened to you is a common experience for all of us. I got razzed during a grading once when I winded my sparring partner..it turned out that she was a junior - 14 years old. However, she had no white stripe on her belt, and I had just thought that she was a young-looking adult 5th kyu. (I was same rank).
Whenever you hurt somebody without meaning too, it is important to take the time to reflect on what happened, and identify what you would do differently next time. You might find that you have something you could express to the parents of the child that would make them feel like you have learned and grown. In my day-job, I manage a hospital. Even when we hurt somebody very,very badly, sharing information about what we have learned and what we would do differently next time helps with the healing. Karate is no different.