We are already God's children because of immense and incomprehensible love, but we are not yet like him the way we will be when he comes. There is yet a purifying work to do. This probably means that, in John's mind, what is impossible for the Christian is a life of unchanged continuation in sin the same as when he was not born of God. In view of all his insistence that Christians do sin, we can't take these verses to mean Christians don't sin at all. We should take them to mean that Christians don't go on sinning without conflict and confession.
Christians see it, hate it, confess it and fight it.
And they do so with increasing vigilance as they grow up into Christ. The point is not that doing good earns eternal life or gets us connected to the life of God in Christ. The point is that a changed life shows you are already connected to God as his child. They don't avoid sin first, in order to get God into their lives. God gets into their lives first, and then they start overcoming sin.