Three questions recur. Accountability concerns which obligations applied to a person and when. Recompense concerns the relation of obedience and wrongdoing to reward or punishment. Divine grace can include a gift not earned as wages for adult performance.
A child may not have performed duties imposed upon adults. That fact alone does not entail that God cannot admit the child to Paradise through grace. Nor can imagined future wrongdoing be treated as an actual sin committed by the child.
Sound argument proceeds in order: identify responsibility and any ground for blame, assess the relevant revealed evidence, then state a conclusion with no more certainty than the proof supports.