What happens to kids who die and never got to form their own beliefs?

In 2 Samuel 12:23 David was comforted by the knowledge that his infant son went to heaven when he died and that he would see him there. Exodus 32:32-33, and Revelation 3:5, among several other verses lead us to believe that everyone is initially written in the Book of Life. After all, according to 1 Timothy 2:4, God does not want anyone to perish. But when we steadfastly refuse God He does not force Himself upon us and we are blotted out of the Book of Life by our own free will. Obviously a small child or a mentally handicapped person who is unable to make these kinds of decisions would then automatically go to heaven by default, since their names remain in the Book of Life.