How to show child pages on Mobile navigation?

Is there some CSS I could add to show the child pages of the parent items in my nav bar when using the mobile menu?

The top-level item for one of my navigation links isn’t a link but rather serves as a header for the sub-pages under it so I need my visitors to be able to see the child pages.

I tried setting the depth of the wp_nav_menu array set to -1 as a workaround, but that breaks the non-mobile menu functionality and I’d prefer to hide the children anyway until the parent is tapped.

Thanks for any help!

-Nick

PS: Does anyone know how to change the menu icon from a single line to the three-line “hamburger?”