The Lord comforted David with the comfort of the covenant. That comfort is that the children of believers are holy, not by nature but in virtue of the covenant of grace, in which they are included with their parents. Therefore, God-fearing parents may believe in the salvation of their children whom God calls out of this life in their infancy. David could pass that comfort on to his wife Bathsheba as he had confessed in verse 2 Samuel 12:23: I shall go to him, but he will not return to me.
That is his confession about heaven.
24 Then David comforted his wife, Bathsheba, and went in to her and lay with her, and she bore a son, and he called his name Solomon. And the LORD loved him