There is a slight theological difference between a son of God and a child of God.
Jesus is the only begotten son of God.
I am a child of God, indeed. So are all who have faith in Jesus.
The book of John tells us that Jesus came and died for us, so that we could be covered in His blood, and we could be grafted into the family tree, even if we were Gentiles.
We have the full privilege of the Jewish adoption, which is why we can approach God with confidence and say , like Jesus says-”Abba” Father- please come to my rescue. And He will. Read the Books of Matthew and Romans.
Now you are talking living and dead. If you have faith in Jesus, you are a child of God, and you a saint, because we know there are no favorites in the Father’s house- we are equal- so you are then equal to a saint.
And if you think about it- a sinner saved by grace is different than a saint who sins.
That is what Paul talks about.
We are the saints. We are the children of God.
Christians ARE sons and daughters of God as well as being heirs to His kingdom.