Before we get to bravery...

Jon said: "In Matthew 18:3 for instance, Jesus says:'I tell you the truth, unless you change and become like little children, you will never enter the kingdom of heaven.' What does it mean to become like little children? I think it might mean that we’re supposed to be as brave as a six year old."

Actually, in the next verse of Matthew 18 we see; "Whoever humbles himself like this child..." So before we get to bravery we must be as humble as a six year old.

Jesus used the small child for this illustration because the apostles constantly pestered Him as they were jockeying for position in this Kingdom He spoke of, wanting to know who would sit at His right and left and what the ranking system would be. What merit of their own would earn them the most favor???... A small child has no merit of their own: no achievements and merits, no status and title, no degree or diploma. They have nothing of themselves to earn favor with the Lord. And whatever "bravery" a child might have generally comes from the ignorance of not having yet learned the risks and consequences of many situations.

Conscious of our sin nature, understanding that all of our works of "righteousness" laid before Christ are like filthy rags (Isaiah 64:6), when we are broken of ourselves, we may approach Jesus with the humility of a small child, not with bravery but in all fear and trembling (Philippians 2:12) because "the Lord takes pleasure in those who fear him, in those who hope in his steadfast love." ~Psalm 147:10-11

"At first there was the fear that this 'terrible storm' and 'awesome terrain' might claim your life. But then you found a refuge and gained the hope that you would be safe. But not everything in the feeling called fear vanished from your heart. Only the life-threatening part. There remained the trembling, the awe, the wonder, the feeling that you would never want to tangle with such a 'storm' or be the adversary of such a power." ~John Piper pg.198, The Pleasures of God

"Courage is not the absence of fear, it is the absence of self" - Erwin McManus

In all the humility of a small child, not representing ourselves but the glory of God and His name we may step out into this world with great bravery.