Permission to edit/add subitems but not parent items

I would like to have a permission setting to allow users to create subitems on projects they’re assigned to but not be able to create parent items. Essentially the parent items is submitted as a project by our client and the Monday user will add subitems as they complete their work on the project. The only way I can give my users access to create subitems is by making them board owners which is somewhat of a security issue. They should not be able to alter the structure of the board, delete columns, access restricted columns but I have to grant that access for subitems.