In the hereafter what is the condition of the children of disbelievers who die before the age of puberty?
In Risale-i Nur Collection, Bediuzzaman writes:
If those who died or perished as a result of a divine calamity, which is brought about by mans own evil deeds, are 15 years old or below, they are martyrs no matter what religion they have adopted. Their huge reward makes their sufferings nothing.
These words take us to the Era of Bliss. While the Prophet Muhammad (PBUH) was explaining the verse, That no bearer of burden shall bear the burden of another. (An-Najm Surah, 53:38) to his companions, he was asked of the condition of the Christians children. As a response, he replied, They will be with their fathers, but only Allah knows how they will be treated. In another report, he told that children of Christians would go to paradise.
In two reports, there is no implication that they will go to hell. Going to hell is one thing; suffering from tortures is quite another thing. Just as Allah Almighty employs some of His angels in paradise and hell, He may place some children in paradise or in hell. But in accordance with His mercy, He will keep them away from hellfire and they will offer thanks to Him for His bounties. On the other hand, they may live a life similar to that of earth somewhere between hell and paradise.
It is strange to wonder if the children of non-Muslims will go to paradise or envy them. What a Muslim needs to do is to seek ways to save them instead waste time on pointless arguments that would have no effect on the result.