Muslim children who do not reach the age of puberty, if Allah the Exalted wills, will be in Paradise, through the Grace of Allah and His Mercy.

Aboo Hurayrah (may Allah be pleased with him) narrates that the Messenger of Allah ? said, “Their young ones roam freely in Jannah ; one of them would meet their father – or he said ‘their parents’ – thereby taking him by the hem of his garment – or he said ‘by his hand’ –as I take them of your garment, whereby they do not take their hand off – or he said ‘they do not stop’ –until Allah enters them both into paradise.”

Anas (may Allah be pleased with him) narrates that the Prophet ? said, “There is not a Muslim person who has three children die before they reach the age of puberty, except that Allah will enter them into paradise, by virtue of His Mercy on them.” Ibn Hajr said, “One who is the cause for shielding his parents from the fire of hell, is more worthy of being shielded from it himself, because they are the reason behind that Mercy and its cause.”
An-Nawawi said, “Those whose views are relied upon from the scholars of the Muslims are in agreement that whoever dies from the children of the Muslims then they are from the people of paradise, due to them not having reached the age of accountability.”