I asked him about the children of the polytheists, and he replied, "Kill them along with them." The narrator added that he had forbidden killing them on the day of Khaybar.
وسألته عن أولاد المشركين فقال اقتلهم معهم قال وقد نهى عنهم يوم خيبر
No references are available.
Its isnad is sahih. This is one of the additions, and it has already been mentioned recently.