Create a Parent from Child using fields from Child then pass New Parent Record ID to Related Parent ID under Child to link them

For what it's worth, I've done similar stuff w/ webhooks. So a webhook on the child table would create the parent record w/ the needed info--include the child record's RID in a "source RID" field or something similar--then, a webhook on the parent table would pick up on the fact that the "source RID" has changed, and modify the child record based on that, linking it to the newly-created parent.