How to get a reference to a child window of an app?

What system?

How is myapp defined?

You might try the following:

for i in range(100):
    r = myapp.window(i)
    if not r: break
    print "window", i
    r.highlight(2)

this will highlight all reachable windows on after the other and report the region aspects with the window number.

myapp is the return value of App('C:\some\path\to\an\exe')

So I tried what you gave me and it turns out that window(0) is what I needed.
My assumptions about how that was working were way off.
Does this mean that if the app's child window opens a child window of its own, does that then become window(0)?
Basically like a fifo stack?

As far as I understand, what they have done on windows in this area, then you get the windows, that are registered with the respective process (whose process id is internally stored when saying x = App('C:\some\path\to\an\exe')), in the current z-order, means frontmost is 0 and so on.
So this has nothing to do with fifo - it is just dynamic with respect to the focus given with keyboard and/or mouse actions.

But one has to be careful with that: some of the windows you get might not be visible at all at that moment.

You might check this with the above test, by making your main app window frontmost (if possible - the dialog window might be modal (blocking clicks on other windows of same app until closed)).

Interesting.
Thanks RaiMan.