8 In other words, it is not the children by physical descent who are God’s children, but it is the children of the promise who are regarded as Abraham’s offspring.

Read Romans 9:8 Using Other Translations

That is, They which are the children of the flesh, these are not the children of God: but the children of the promise are counted for the seed.
This means that it is not the children of the flesh who are the children of God, but the children of the promise are counted as offspring.
This means that Abraham’s physical descendants are not necessarily children of God. Only the children of the promise are considered to be Abraham’s children.