When I was nine years old, I stole some money from my martial arts teacher (I hadn't reached puberty at the time). I didn't know it was a major sin. Now that I'm older, I found out that I could not be forgiven unless that person forgives me and I return to her that money. The problem is I have no way to find her again and I really don't know what to do and I'm very scared that Allah will not forgive me and will send me to hellfire.
Your former martial arts teacher is not the only person in the world. It is good that you realized you did something wrong. If you cannot find him, though, do some good deeds to other people, even related to your sin. Explain to some Kids that don't know it that stealing is bad. Give some money to people in need.
Even though I don't know your teacher, I am sure that he has already forgiven you. People working with children know that children can act in a completely irrationnal and immoral way without realizing it. It is true for every teacher I met. He would not teach martial arts to Kids if he was not like this.
And one last thing, life is easier and healthier when you don't think about hell all the time. If you do, you won't enjoy it much.