1) Child soldiers
They are sometimes forced to become child soldiers and fight - even when they don't understand what the war's about.
In Liberia, children as young as seven have been found fighting in combat.
Sometimes children are taken from their families and forced to fight.
Or they may be forced to fight to protect their families.
2) Destroys families
Children often get separated from their family because of war - or lose their parents altogether.
Many children are sent away from home if the fighting is really bad.
And their fathers and brothers go off to war, and may never return.
In Rwanda, 300,000 children have no mum or dad and have to run their own family, because their parents were killed in a civil war in 1993.