We know that 1/3 of the dead are children, according to the headline. We also know that children make up about half the population of Gaza. We assume that none of the combatants are children.
If a person is killed, that person is either an adult combatant, an adult civilian, or a child civilian. Since child civilians make up 1/3 of the dead and there are as many adult civilians as child civilians in Gaza, adult civilians therefore make up another 1/3 of the dead. That adds up to 2/3 of the dead being civilians. 2/3 civilian dead and 1/3 combatant dead is a 2:1 ratio of civilians to combatants killed.
Let me try to explain it another way.
We know that 1/3 of the dead are children, according to the headline. We also know that children make up about half the population of Gaza. We assume that none of the combatants are children.
If a person is killed, that person is either an adult combatant, an adult civilian, or a child civilian. Since child civilians make up 1/3 of the dead and there are as many adult civilians as child civilians in Gaza, adult civilians therefore make up another 1/3 of the dead. That adds up to 2/3 of the dead being civilians. 2/3 civilian dead and 1/3 combatant dead is a 2:1 ratio of civilians to combatants killed.