If you access the iPad volume control from the Control Centre, the volume control slider will remain visible on screen until the Control Centre is dismissed from view. The Control Centre is exposed using a downward swipe from the top-right corner of the screen.

More information about using the Control Centre can be found within the often-overlooked iPad User Guide. Here is a direct link to the relevant section of the Guide:
Use and customize Control Center on iPad - Apple Support
Instead of using on-screen volume controls, why not simply use the physical volume control buttons located on the edge of the iPad?
Adjust the volume and silence on iPad - Apple Support