What is the ruling on supplicating Allaah to have mercy on non-Muslim children who have died before reaching puberty?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.

The most valid opinion on this issue is that which was mentioned by the Prophet, sallallaahu ‘alayhi wa sallam, when asked about the destiny of non-Muslim children who die before reaching puberty. He, sallallaahu ‘alayhi wa sallam, mentioned that only Allaah Knows what they would have done if they had reached puberty and whether they would have been believers or disbelievers. [Muslim]

Furthermore, Ibn Taymiyyah referred to a narration on the authority of Abu Hurayrah where the Prophet, sallallaahu ‘alayhi wa sallam, mentioned that on the Day of Judgment Allaah will send them a messenger: whoever follows him will be admitted to Paradise and whoever disobeys him will be thrown into Hell. Only then, what Allaah Knows about them will appear, which is their belief or disbelief, and not just rewarding them or punishing them due to His Knowledge about what they will do.

Therefore, it is impermissible to supplicate Allaah to forgive them because one does not know where they will go; to Paradise or Hell. It is not permissible to supplicate Allaah to forgive a non-Muslim, as indicated in the Quran and Sunnah. Had this been permissible, this would have been reported to us that the companions and the generations which followed them did so, but not even a word was reported on their authority about this.

Allaah Knows best.