Saying that we are “children of God” and “heirs” at the same time is a wonderful thing. Imagine for a moment that the president of your country decides to be your best friend, the first thing you would think is why you, that you do not deserve it, etc. So this is much bigger, because it’s not about a president or famous person, it’s about the most imaginable being that exists.
We are children and heirs of God, and so the Bible describes it:
15 For ye have not received the spirit of bondage again to fear; but ye have received the Spirit of adoption, whereby we cry, Abba, Father.
16 The Spirit itself beareth witness with our spirit, that we are the children of God:
17 And if children, then heirs; heirs of God, and joint-heirs with Christ; if so be that we suffer with him, that we may be also glorified together.