Open up header.php and find the shortcode [nivoslider slug="slider"]. This is where you can play around with how this is implemented. Introducing a child theme (https://startupwp.com/topic/child-theming/) at this point if you haven’t already might be a good idea, but might also cause other hurtles.
If this needs to be different on multiple sites in a multisite install, but having just the one slider ID is conflicting and you need to use multiple IDs dynamically, we’ll need to implement a PHP if statement.
Before we dig in, are we understanding the issue correctly and how many sites is this intended to support, a few, unlimited? Any other details that might help are appreciated.
Thank you.