Whisk the flour, sugar, baking powder, baking soda, salt, cinnamon, and nutmeg in a medium bowl to blend.
Using a fork, coarsely mash the bananas in a large bowl.
Stir in the yogurt, oil, and egg white.
Add the dry ingredients to the yogurt mixture and stir just until blended.
Stir in the cereal. The batter will be thick.
Spoon the batter into the prepared cups.
Sprinkle the streusel over, gently pressing it into the top of the batter.
Bake until a tester inserted into the center of the muffins comes out clean, about 25 minutes.
Cool 5 minutes in the pan on a cooling rack.
Remove the muffins from the pan and cool completely on the rack.
The muffins can be made 1 day ahead. Store airtight at room temperature.