Scripture implicitly affirms the concept that we call the age of accountability (Deuteronomy 1:39; Isaiah 7:15-16). This is relative due to the fact that everybody develops differently. Thus, there exists a period of time in childhood in which God in His mercy does not count sins against a person due to lacking reason and understanding. While babies are born with a sin nature, they still cannot actually commit acts of evil.
The God of the Bible is a God of love, mercy, righteousness, and truth. He would not condemn to hell somebody who never even had a chance to live. We must also keep in mind those who have severe cognitive disabilities. Though it is a teaching of Scripture that God has inscribed a moral law into the hearts of men, every person needs to have the mental capacity to act. Each man will be judged according to his own conduct (2 Corinthians 5:10; 1 Peter 1:17). Babies cannot do anything on their own. So what could God judge them for?