There is a a sect/organization (let's call it Avril for now) that is one of the strongest in the world. Every 4 years they will hold a recruitment which us not a recruitment but a kidnapping. So they 'recruit' 10 kids from all over the world which show good potential. The kids are no older than 10. So they take the kids and put them through all sorts of training using all available resources. They are trained individually according to their attributes. If they have a powerful body, then their minds are tempered to cover their weaknesses. Of course, their body will be strengthened as well(in this case). Because of this those trained by Avril are known to be the best of the best.
You would think that they brainwash the youths to grow more powerful. But they actually return then to their families after 4 years. At the age of 15 they are sent a letter that asked them if they want to return to the organization that nurchered them. Only a small number of them will return but the rest would feel guilty and come to the aid of the organization when they were troubled.
Anyway, since people want their children to be strong they showoff their children's skills and hope to attract the Avril's attention. This applied to the (incert family name) as well. And they got what they wanted. Their child was taken on selection day.
However it was not Avril that took him but a group of mercenaries who were hired to kidnap and ransom the child. They sent a note to the family to meet them in a specific location if they want their child back. The family thought it was Avril (it was not signed) and did not bother to since they wanted Avril to train their child (they assumed the letter was standard procedure).
The mercenaries were quite confused and did not know what to do with the child. The head of the mercenaries had taken an interest in the child's talent so he personally raised the child to be his successor. The child was only 3 years old when he was taken and soon forgot about his old family.
After 4 years...
The child's family were quite confused why their son was not back yet so they decided to contact Avril. They were told that Avril did not take their child and they instead wanted to wait 4 more years before taking him at the age of 7 so that he would understand things better.
Knowing this, the family tried everything in their power to find their son but it happened too long ago and they did not find anything.
Meanwhile in the mercenary group...
The boy had grown up thinking that he was a child the head mercenary had found abandoned and generously taken in. He was trained to be a mercenary and was publicly known was one of the best despite his age.
Five years later...
The family had a second child, a girl, who just turned 10. To insure her safety, they decided to hire someone to protect her at all times. They had heard of the power of a young mercenary and decided to hire him since they thought that the could get along better because their age was closer.
Of course, the one they hired was their long lost son.