The Bible does not give us a definite answer regarding the state of little children who are too young to understand the gospel and trust in Christ, but based on certain principles of Scripture, many believe (and I am one of them) children who are too young to grasp the issues of the gospel will be in heaven. The reason is simply this. Jesus Christ, God’s Son, died on the cross for the sin of the whole world (1 John 2:2; 1 Tim. 4:2). He and He alone is the Savior of all men, and the means of receiving that salvation is by personal faith in Christ. But this can only really apply to those who can believe. Since Christ died for all our sin, the only sin that can keep a person separated from God is the sin of unbelief or rejection of Christ. Listen to the passage below which speaks about the convicting ministry of the Holy Spirit in regard to the person of Christ:
And He, when He comes, will convict the world concerning sin, and righteousness, and judgment; 9 concerning sin, because they do not believe in Me; 10 and concerning righteousness, because I go to the Father, and you no longer behold Me; 11 and concerning judgment, because the ruler of this world has been judged (John 16:8-11).
Notice that John said, “concerning sin, because they do not believe in Me.” All people are sinners and must recognize this, “for all have sinned and come short of the glory of God” (Rom. 3:23), but no one is saved by giving up their sins (works) (Eph. 2:8-9; Tit. 3:5). They are saved by faith in Christ’s work who alone could and did pay the penalty for our sin. So, the greatest and basic sin is unbelief. Jesus’ return to the Father will vindicate His righteous life and the truthfulness of all He said, which includes John 14:6. Further, His statement about the fact of the ruler of this world being judged shows that at the cross Christ triumphed over Satan. This also serves notice of the judgment to come on all unbelievers (those who refuse to believe in Christ). Satan’s judgment proves the judgment of all who do not believe in Christ.
But a baby cannot understand that he or she is a sinner nor can they reject Christ. So, since Christ paid for their sin by His death on the Cross, many believe that little children will be in heaven. Also, remember that when David lost his little child who died before the age of accountability, David stopped his mourning and said, “he can’t come to me, but I will go to him.” This suggests that David believed that one day he would see his child in heaven (see 2 Sam. 12:23). While this verse does not prove this concept, it certainly lends support since David seemed to be comforted by this thought.
Let me point out, however, that not all Christians and Bible teachers believe this about children and will simply say, we cannot know the answer to this since the Bible does not give us specific answers. Others deny this on the basis of their belief in election. While others would say the only the children of saved parents will go to heaven since they are sanctified by the believing parent (see 1 Cor. 7).