The eldest is providing an example for the others. It is his responsibility to be a good example. If the player starts using his PC to antagonize the other players (who are probably too young to emotionally separate player and PC), he needs a kind, but firm discussion alone.
If he chooses not to pull it together, there is the in game option of the GM hammer ("Ooh, the ogre hits you again") and finally, a kind, but firm boot from the game.
Also, I would posit that it is worth having a different "GM philosophy" for adults and one for kids. You've stepped into added responsibility and need to adjust your style accordingly, rather than let kids stab each other with sharp (emotional) sticks, ruining the experience of RPGs altogether.
If he chooses not to pull it together, there is the in game option of the GM hammer ("Ooh, the ogre hits you again") and finally, a kind, but firm boot from the game.
Also, I would posit that it is worth having a different "GM philosophy" for adults and one for kids. You've stepped into added responsibility and need to adjust your style accordingly, rather than let kids stab each other with sharp (emotional) sticks, ruining the experience of RPGs altogether.