Most theologians believe that yes, young children and infants do go to heaven if they die. Here’s why. Even though infants and young children are born into sin through Adam, they are not old enough to understand what sin is and how that is rebelling against a holy God. They are not consciously rejecting God’s plan of salvation. They are saved, not because they are without sin but because of God’s mercy and righteous judgement.
And He (Jesus) said; “I tell you the truth, unless you change and become like little children, you will never enter the kingdom of heaven. Therefore, whoever humbles himself like this child is the greatest in the kingdom of heaven”. – Matthew 18:3-4
The idea of “the age of accountability” is a thought of the early church, not the Bible. God is a good judge and He knows the age when a child is old enough to know and understand God’s gift of salvation or to reject that gift. Allow God to be God.
The idea of the need for infants to be water baptized for salvation is a mistaken view of baptism and puts the work of salvation in the hands of people not God.
Infants and young children are too young to consciously reject God, thus God will except them.