[SalesForce] Create List View Quick Action that calls a flow

I think no matter what kind of Quick Action you do, native or customized, it will always open a modal window in Lightning.

You could create a small VisualForce page or Lightning component to add directly to the Lightning page layout, which simply hosts a button that carries out the desired action. That may be the only way to put a one-click action inside the layout.

Here is another thought: Create a native Quick Action that creates a record copying over whatever fields you want as Predefined Values, and allows them to edit any they want to. This will display as a modal dialog, replacing the Edit page they'd get patched through to. But also set a Predefined Value in text field that is hidden from the Action layout. This hidden value would be the ID of the record you clone. Now you can set an Apex trigger that states, if a record is created with a value in the hidden field, run your custom code that clones the child objects.

It's by design. As outlined in Set Up Mass Quick Actions:

Mass quick action lets your users edit up to 200 records in a list view. You can perform mass quick actions on create and update quick actions on cases, leads, and custom objects in Lightning Experience. To set up mass quick actions, you customize the object’s search layout.

So, for now, only Leads, Cases, and Custom Objects are supported. Future releases may include other objects, but when or what they are have not been listed.