1703. Narrated Aboo Huraira (radhi Allaahu anhu) :
The Prophet (Sallallaahu Alaihi Wasallam) was asked about the offspring of Al-Mushrikoon (*); so he said, “Allaah knows what sort of deeds they would have done.”
(*) [H.1703] & [H. 1704] Al-Mushrikoon : polytheists, pagans, idolaters, and disbelievers in the oneness of Allaah and His
Messenger Muhammad (Sallallaahu Alaihi Wasallam).
[Saheeh Al-Bukhaari, Hadeeth No. 466, Vol. 2]
Source : The Book of Al-Qadar : Chapter 6 – Every child is born with a true faith of Islaam (i.e. worship none but Allaah Alone), and what is the decision about the (fate of) dead children of disbelievers and of Muslims. Al-Lulu Wal-Marjan, Compiled by : Fuwad Abdul-Baqi, Translated by : Dr. Muhammad Muhsin Khan
Read The Children of the Disbelievers – Permanent Committee for Research and Verdicts