That He has a perfect plan for their lives. Most grown ups don't know these things. They have not learned that God wants to be tested about His promises. We can't test Him if we don't know the promises. We should tell them that God has a Perfect Will and a Permissive Will.
Miracles are real.
God's Miracle Power is one blessing only He can give us. His Sustaining Power is another and probably more important for them to learn. Character building and molding are a part of God's love for us. Let us not sell our kids short by assuming they cannot learn these things. God says we are to bring them up memorizing and hiding His Words on their hearts.