Due to the nature of polymorphism and Eloquent's hard-coded mapping of model names in the database, setting relationships for child models that inherit permissions of the parent can be difficult (even near impossible depending on app requirements, especially when attempting to do inverse mappings). However, one thing you might consider if you need the child model to never have its own permissions/roles but to only use its parent's permissions/roles, is to override the getMorphClass method on the model.
eg: This could be useful, but only if you're willing to give up the child's independence for roles/permissions:
You can add your own migrations to make changes to the role/permission tables, as you would for adding/changing fields in any other tables in your Laravel project.
Following that, you can add any necessary logic for interacting with those fields into your custom/extended Models.