We are God's children now. Not someday, not when we're good enough, now. John emphasizes this emphatically. At the same time he says: What we will be has not yet appeared. So there is both: a firm present and an open future.
The now is secured. You are a child of God. The tomorrow is exciting because God has prepared more than you can imagine. But you don't need to wait for tomorrow to be sure. Your identity stands now.
Today you are God's child. Completely. And the best is yet to come.