King James Version
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.English Standard Version
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.New International Version
In other words, it is not the natural children who are God's children, but it is the children of the promise who are regarded as Abraham's offspring.