MacArthur, John. 2003. Safe in the Arms of God.
Written for those that have lost children (either in or out of the womb), Safe in the Arms of God seeks to expand John MacArthur's theology of "Instant Heaven." It's a question he (and many other pastors and believers in general) have been asked. What happens when a baby dies? His response is and always has been "Instant Heaven." He says this not because he wants to give false assurance to grieving parents and their families, but because he believes that this is what the Word of God teaches.
Theologically, the question is what happens to those who die at an age that they are unaccountable. To those that mentally (physically, emotionally, psychologically) are unable to comprehend the gospel message, unable to have the faith to believe. In other words those that can neither receive or reject the gospel truths. (And when you think about it, there is a lot to grasp. The awareness of sin. The separation from God. The need for a Savior. The realization that Jesus, the very son of God, bridges the gap between sinful man and a holy God. The acceptance of Jesus as your Savior, as your Lord.
John MacArthur's answer may shock some. (It may not shock others.) Some may feel that the 'soul' of the baby is dependent on the faith of the parents. But MacArthur asserts that this just isn't so. He believes, and he argues within these pages, that each and every baby (and/or young child) that dies is welcomed by God into heaven. This is irrefutably good news for believers that have experienced loss in their lives. For the believer, the reunion is just a matter of time. You will see your child again. In the meanwhile, while the loss is difficult to accept, there is much comfort to be grasped in the knowledge that their child is in fact in heaven.
The book also highlights how tremendous a place heaven is. While parents may be sad, there is confidence that their child is anything but. It may sound trite, but heaven is a wonderful place. A place where there are no tears, no sadness, no pain.
Using the Bible as the basis for his theology, MacArthur argues his case quite well.
On a slightly related note, I'd suggest musically listening to "With Hope" by Steven Curtis Chapman from the Speechless album. And "Lullaby" by Andrew Peterson from the Walk album.