If Adam was the son of God, wouldn't his children also be grandson's of God?
Nowhere in the Word of God do we see the phrase, "grandsons on God." The phrase, "son of God" is used of angels, and of Adam (because he was a direct creation of God). After Adam and Eve, being told to "be fruitful and multiply", bringing forth children after their likeness ...these offspring are "children (sons/daughters) of men."
Notice, Jesus was called the Son of God AND the Son of man. He was/is God's Son, but came here and was born in the likeness of sinful flesh. Once we are born again through faith in Christ, we are given "power to become the sons of God." We are a new creation in Christ. But prior to the new birth, we are just children of men.