Thank you for contacting us here in the Support Forum and for providing the Debug Informations
It would be good that you first update your Toolset to it's latest version. Can you do this please?
❌ Be sure to backup your database first before you proceed! ❌
⌥ You can use a plugin for this if you like.
I often use the Duplicator plugin for this purpose.
See: http://wordpress.org/plugins/duplicator/
I understand you have the Post Type A with just one parent Post Type B, and the Post Type A has also a Child Post Type (MtM).
You can always edit, with CRED, in the Child Post Type the parent Post.
(as you mention, it's already embedded)
But you can't add Child Posts from the CRED form that edits (or creates) parent posts.
You need to either use a Child CRED form link, or edit/create the Child with CRED and set the parent.
https://toolset.com/documentation/user-guides/cred-forms-for-child-content/
You could use a cred_save_data hook to insert/update your Child posts with the parent Post ID once the CRED Form that edits the Parent is submitted.
https://toolset.com/documentation/user-guides/cred-api/
This requires some PHP Code approach.
It depends whether your Child Posts (MtM) already exist, or they need to create them first as well?
However, it's not a trivial approach, as you need to get the Child Posts from somewhere first, and then update the _wpcf_belongs_parent_id Custom Field, so to set the (new) parent ID for each Child.
I need further informations how exactly the process is and if the data (child Posts) are already available, otherwise, you would also need to insert those posts first (in your CRED save_data_action) with wp_insert_post.
Please could you provide me the additional Infos?
Thank you