FWIW we are experiencing the exact same issue. We would like a widget to appear on one child page of a parent with many children. We used “Show on Selected” + Target by URL (tried with the slug *series/amazon and with the full url) + Exclude by URL, with the URL of every single other child of the */series/* page that the widget should not appear on. The widget appears everywhere.
If the plugin worked, it would be exactly what we need.
