For what it’s worth, King David had definite certainty that he’d see his son in heaven. In 2 Samuel 12:15-25 his son is born very ill; David weeps, fasts, and pleads with God to grant healing, but the child passes away on the seventh day (before a child would customarily be baptized/circumcised).
Yet when the child passes away, David immediately stopped grieving, "…arose from the earth and washed and anointed himself and changed his clothes. And he went into the house of the LORD and worshiped. He then went to his own house. And when he asked, they set food before him, and he ate. Then his servants said to him, “What is this thing that you have done? You fasted and wept for the child while he was alive; but when the child died, you arose and ate food.” He said, “While the child was still alive, I fasted and wept, for I said, ‘Who knows whether the LORD will be gracious to me, that the child may live?’ But now he is dead. Why should I fast? Can I bring him back again? I shall go to him, but he will not return to me.”