This is understandably a very sensitive and tender topic. I believe the ultimate answer comes to us from Romans 5:20 where Paul says that where sin abounded, grace superabounded. I take it to mean that Christ not only conquers sin, he goes far beyond what sin could do. Grace always surpasses sin. Because of that, I believe that those who either lack the mental capacity to believe or who die before they can understand the gospel will be in heaven by the grace of God. To be sure, this is an inference, not an absolute statement. But if grace triumphs over sin, then I believe God in his mercy will redeem the little ones who never lived long enough to understand the gospel.