When I use Snoop, I always first start snoop and then use the "shift+control" shortcut to focus on what I want. This seems to work even if I do it to a child window. So try this:
- Start application, open child window
- Run Snoop
- Snoop any window in the application
- Focus on your child window (click on the title bar of the window)
- Move the mouse over what you want to inspect, and hold down the Shift and Control keys simultaneously
- A red outline appears in your application, and you should see Snoop show/select the element in question in the tree. If you look up the visual tree on the left you will now see the child window is the parent of the tree.
This solution was last tested in Snoop 2.8.