I gave an example here for removing layouts https://support.advancedcustomfields.com/forums/topic/disable-layout-from-showing-in-template/
To add layouts you would need to have the layouts saved in a JSON file or you would need to create them in PHP. You can then load this field group and append the layouts from the flex field to the flex field of the parent theme group.
I’m really not sure how you could manage this because there will be no way for the parent theme to know what field group to merge layout from. This is probably something that would need to be coded into each child theme separately.
