We are their children (descendants), and God has made this promise come true for us. God did this by raising Jesus from death. We also read about this in Psalm 2:
Now I will tell you about the Lord’s agreement.\par The Lord said to me,\par “Today I become your father! {\cf2\super [4]} \par And you are my son.\par
After Jesus was raised from death, his followers remembered that Jesus had said this. So his followers believed the Scripture {\cf2\super [42]} \{about him\}, and they believed the words Jesus said.)