I think Moodysson avoided to use popular or professional actors of all ages to play the actors in this movie.Do you think that the movie would be the same without the local actors?
He tried to create as close atmosphere as like it was reality.And i am sure he did it.And by participating local amateur actors this increases the originality of the film.
All these kids playing in this movie/''amateur'' actors why haven't participated in something else?All these kids disappeared as they appeared suddenly in the cinema world.