I don’t think you’ll be seeing a parent post or child post location rule any time in the near future built into ACF, especially for custom post types, and even more so of the parents/children are not the same post type. You’re going to need custom location rules https://www.advancedcustomfields.com/resources/custom-location-rules/.
I have also created several examples of custom location rules in this repo https://github.com/Hube2/acf-filters-and-functions. I don’t have one specifically for post parent, but there are a couple there that could be easily modified to be specific to the post parent.
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