This is what you need:
https://github.com/AdvancedCustomFields/acf/blob/master/includes/locations/class-acf-location-page-parent.php
It allows you to specify an exact page parent (Parent XYZ) in your case. You can create a field group, select to show if Page Parent = Parent XYZ.
I’ve had pro for so long I just don’t remember what isn’t in the free version so I suggested if you don’t see Page Parent, it could just be in the pro version, but that’s the rule that does the exact thing you’re asking for. The fact it’s in the repo for the free version suggests it isn’t a pro-only feature. I’d double-check your location rules. Best of luck!
