1 Jn 5:1, Jn 1:12, Gal 3:29, Gal 4:28
Abraham's physical children are not God's children. The children of God's promise are the children of God. The children of God's promise are those who believe in Jesus as the Christ. (Rom 9:6-13, 1 Jn 5:1, Jn 1:12)
Rom 9:6 Not as though the word of God hath taken none effect. For they are not all Israel, which are of Israel: 7 Neither, because they are the seed of Abraham, are they all children: but, In Isaac shall thy seed be called. 8 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. 9 For this is the word of promise, At this time will I come, and Sarah shall have a son. 10 And not only this; but when Rebecca also had conceived by one, even by our father Isaac; 11 (For the children being not yet born, neither having done any good or evil, that the purpose of God according to election might stand, not of works, but of him that calleth;) 12 It was said unto her, The elder shall serve the younger. 13 As it is written, Jacob have I loved, but Esau have I hated.
1 Jn 5:1 Whosoever believeth that Jesus is the Christ is born of God: and every one that loveth him that begat loveth him also that is begotten of him.
Jn 1:12 But as many as received him, to them gave he power to become the sons of God, even to them that believe on his name:
If we believe in Jesus as the Christ, we are the descendants of Abraham. And we are the heirs of Abraham. (Gal 3:29, Gal 4:28)
Gal 3:29 And if ye be Christ's, then are ye Abraham's seed, and heirs according to the promise.
Gal 4:28 Now we, brethren, as Isaac was, are the children of promise.