I am trying to assign a parent post to multiple child posts.
I have a parent custom post type and child custom post type set up. If I individually click on a child post I can scroll down and select which parent post it belongs to (as per your instructions that I have read about creating post type relationships), but I need to do this in bulk.
Is there a way to select multiple child posts and assign them all to the one parent post at once?
This is not possible within Types, you will need set it's parent post in individual child post.
And Types is using a custom field "_wpcf_belongs_[parent-slug]_id" to store the parent post ID, for example the parent post type is using slug "parent-cpt", then custom field slug is "_wpcf_belongs_parent-cpt_id", you can try assign multiple posts parent post by wordpress function: