Reference: Silsilah Kitaab ad-Da ‘wah (11), al-Fataawa – Volume 4, Page 7
Question: A child is born of disbelieving parents and dies before he attains the age of responsibility/understanding; Is he a Muslim according to [the Law of] Allaah or not, bearing in mind that the Messeger of Allaah ﷺ said:
…the hadeeth. And if the child was a Muslim is it obligatory upon the Muslims to wash him and pray the janaazah prayer over him? [Please] advise us, and may Allaah reward you.
Response: If one who has not reached the age of understanding dies whilst under the care of his disbelieving parents, then his ruling is the ruling of his parents with respect to the affairs of the world. So, he is not to be washed, nor prayed upon and nor is he to be buried in the graveyard of the Muslims.
As for the Hereafter, then his affair is with Allaah (Subhaanahu wa Ta’aala). It has been authenticated that when the Messenger of Allaah ﷺ was asked about the children of the mushrikeen, he said:
And some of the people of knowledge have said that Allaah’s (Subhaanahu wa Ta’aala) decree will be made clear on the Day of Resurrection, where they will be tested just as the people of fitrah and their like will be tested. So if they respond to that which is requested of them, then they will enter Paradise. And if they abstain [from responding] then they will enter the Fire [of Hell].
And ahaadeeth have been authenticated with regard to the testing of the people of fitrah on the Day of Resurrection, and they are those to whom the da’wah of the Messenger ﷺ had not been conveyed, and those upon whom this ruling applies from the children of the mushrikeen, as Allaah (Subhaanahu wa Ta’aala) says:
And this is the most correct of the opinions regarding the people of the fitrah and their like from amongst those to whom the [Islaamic] da’wah had not been conveyed, and this is the opinion of Shaykh al-Islaam Ibn Taymiyyah and his student Ibn al-Qayyim, in addition to a group from amongst the Pious Predecessors and those who came after them (rahima-humullaah).
And Ibn al-Qayyim (rahima-hullaah) has covered the topic of the ruling of the children of the mushrikeen, and the people of the fitrah at the end of his book: “Tareeq al-Hijratayn” under the title: “Tabaqaat al-Mukallifeen”.