What you’re asking for is support for updating a collection of parent objects with a child (or perhaps multiple children). Is that correct?
Yes, exactly
What you’re asking (if I understood correctly) is not possible with a single query
I did not realize
OK no worries, will create loop 
Hi Mark, also chiming in, because I am very surprised by what you are stating.
As said above, the needed operation could be performed in a single call in a standard SQL database. Is there not a way for you to arrange it so that it is a single API call, and then a batch update behind that ?
Relationships are set in a different way than regular column properties. As a result, a single query to update relationships for multiple primordial objects is not possible.