where does the Bible say that?
Jesus indicated that people who enter heaven are like children, implying that children go to heaven.
And He called a child to Himself and set him before them, and said, “Truly I say to you, unless you are converted and become like children, you will not enter the kingdom of heaven.” ~ Matthew 18:2-3
Then some children were brought to Him so that He might lay His hands on them and pray; and the disciples rebuked them. But Jesus said, “Let the children alone, and do not hinder them from coming to Me; for the kingdom of heaven belongs to such as these.” ~ Matthew 19:13-14
2 Samuel 12:23 (David speaking about the baby he had with Bathsheba.)
And Paul in Romans 7:9 (children before the age of accountability.)