Contrary to popular belief, no one is born a child of God. Jesus told the unbelieving Jews they were of their Father the devil, and Paul says to the Ephesians that they were once the children of wrath. Thus, if we are to be a child of God, we must experience a spiritual adoption, and this spiritual adoption is what takes place the moment we trust in Christ for salvation.
As Paul says in Galatians 3:26, “For ye are all the children of God by faith in Christ Jesus.” So it doesn’t occur gradually, it is instantaneous upon genuine conversion.
It also provides immediate benefits, such as confidence in prayer and the promise of daily provision when we seek first the Kingdom of God, as well as the future guarantee of glorification in Heaven. The only question is, are you a child of God?