When Dragon said to Kuma that children are their parent's weak point, I don't think he is only referring to himself or that he'd known about Rocks and Teach. I think he's also referring to himself and his father. Because think about this, Garp is a Marine who will openly insult a Celestial Dragon but during the God Valley incident, he chose to protect them. And I don't think he would do that not unless the CD has a leverage against him. And right now, who is in God Valley, defying the CDs who means a lot to Garp and whose situation can be used against him?
Right, his own son, Dragon.