It is said that God created the world in his own image, and that we are children of God because humans have also been created in God's image. In case you're wondering, the first part is undoubtedly true: of course God created the world in his image - how else would he create it? The whole world is an expression of God, and by getting to know the world within us and outside us we can get closer to God.
As for the second part; even through we're created in God's image, are we children of God? This is a two-part concept. Compared to God (or just emotionally and spiritually more advanced societies), we are indeed children. We are also an image of God, since that is the basic reality of the universe. By combining these two concepts we get that we are 'children of God'.
There is nothing wrong with being a child, but an adult who refuses to grow up is not a child, but an irresponsible adult. In this sense it is quite misleading to call ourselves children of God. It is time to mature, grow up and be co-creators with God - instead of nagging on him and blaming him for our own deficiencies. While being oblivious to the mysteries of the world is a boon to a small child, being selfish and stubbornly ignorant is a huge character flaw for an adult.
We are children of God, but children grow up. We as individuals and humanity as a whole have to grow up too. To do that we have to open our hearts to the Love of God and grow a backbone. Being selfish and self-destructive just doesn't cut it.