Even as an adult, Vito preferred that people underestimate him. Look at the scene with Signor Roberto or Fanucci. He liked to understand people, letting them show their true colors.

Plus, if you were a little kid, and your mom took you to see the big Mafia chief that had murdered your father and your big brother, wouldn't you be a bit intimidated?? Perhaps even quiet?

And remember that Don Ciccio had it exactly right - when he grew, Vito DID come back for revenge!