Sahih Muslim 2660
Sahih Muslim · Book 46: The Book of Destiny · The Meaning Of "Every Child Is Born In A State Of Fitrah" And The Ruling On The Dead Children Of The Disbelievers And Of The Muslims.
Abu Huraira reported that Allahs Messenger (way peace be upon him) was asked about the children of the polytheists who die young. Thereupon Allah's Apostle (ﷺ) said:
حَدَّثَنَا ابْنُ أَبِي عُمَرَ ، حَدَّثَنَا سُفْيَانُ ، عَنْ أَبِي الزِّنَادِ ، عَنِ الأَعْرَجِ ، عَنْ أَبِي هُرَيْرَةَ ، قَالَ سُئِلَ رَسُولُ اللَّهِ صلى الله عليه وسلم عَنْ أَطْفَالِ الْمُشْرِكِينَ مَنْ يَمُوتُ مِنْهُمْ صَغِيرًا فَقَالَ " اللَّهُ أَعْلَمُ بِمَا كَانُوا عَامِلِينَ " .
It is Allah Who knows what they would be doing.