Background : In a certain medieval world with magic, people are always in danger of being attacked by non-human races, like orcs, trolls or giant spiders, etc. Villagers keep the village safe by using swords or magic. However the frequency of the attacks on humans is increasing. Therefore the King wants to choose children who are talented in magic, and give them training to repel the non-human invaders. Magicians are always more powerful than swordsmen, so it makes sense to train a group of magicians rather than a army of soldiers. (An average magician can take on 15-20 soldiers.)
The problem is, how should he convince the parents, that he needs to take their children away, in order to train them to become monster-slayers? It's widely known, that becoming a magician is extremely difficult, especially those who specialise in killing.
Note : The children would be taken from their parents at the age of 8-10 years old, and they would undergo training until he/she is 20. Then they would wander to different villages, protecting them from attacks. Children with a talent in magic are very rare, in average 1 out of 500 (which is very rare in the medieval ages). The children can be taken by force, but that might cause a revolt (the king does not want to be known as a tyrant, and even one child taken would cause the entire village to know which might spread to other villages). All people in this world worship one main god, the Sun God. Also, humanity is quite weak, so building a large army is almost pointless, since they don't know where the monsters live, attacking the non-human races is not a possibility.