More natural way
I thought about this too in the beginning (see this thread).
I tried to query for |?has subobject.PropertyOfParent but this didn't work.
Maybe I would have to create a subobject property holding the parent page and then query for that one.
Can someone try it and adapt the page? :)
Advantage of my proposed solution: It doesn't need another property.
Yes, I meant creating a property for subobject to backreference the parent page, then querying
?MyParentPage.PropertyOfParent. That's SIO way and in many cases such property is more meaningful than the property pointing from the parent page to the subobject. The backreference property may be universal or specific to the subobject class.