Drush commands in submodule

Question

I have a module as defined:

parent
    child1
    child2
    ... parent module files

These modules install and operate correctly. My question is regarding drush. I want child1 to have the drush commands. But when I declare them in child1 drush does not recognize them. However, when I declare them in parent, drush finds the commands and I can use them.

Looking through the drush topic docs-configuration file to see what that might afford me, I could find nothing to tell drush to look at child-module folders.

And the drush documentation is.... lacking.

Is it possible to define drush commands in child1 or must they be defined in parent?