This question can’t be answered by Scripture with full clarity so we offer our guidance and an opinion.  We believe the sacrifice of Jesus covers the original sin of children (and the mentally disabled) until they reach an age of accountability.  This age can vary from person to person, but usually falls within the teen years. It’s a time of life in which a child moves to a place of responsibility and can be accountable for their decision of whether to accept Christ. After that point, the only assurance of heaven is by personal salvation in Jesus Christ.