Yazid bin Hurmaz says that once Najdah bin Amir asked Sayyiduna Ibn Abbas (may Allah be pleased with them both) about killing children. He replied in writing: You have asked me whether the Prophet (peace and blessings be upon him) ever killed any child among the polytheists? So remember! The Prophet (peace and blessings be upon him) did not kill any of their children, and you should not kill anyone either. However, if you come to know about a child in the same way as Khidr (peace be upon him) came to know about that child whom he killed, then that is a different matter (and this is not possible for you).