Contrary to popular opinion, we are NOT ALL children of God. We are all created by God, but due to sin we are estranged from God and are ‘children of wrath’ (Eph 2:1-3). If the Bible is true, then it is a mistake to say, ‘We’re all God’s children.’
Contrary to our own unbelief, we can BECOME children of God. We have sinned and broken God’s law, we deserve his punishment, but none of us is too far gone to experience God’s saving grace: in great love God sent His Son to become a human and save us from the power and penalty of sin. If we believe in Him, we will be saved (John 3:16) and BECOME a child of God (John 1:12-13).