Do I need a Child theme to use Divi Den Pro?

Using a child theme is not required for Divi Den Pro to work. You can use the parent Divi theme only.

However, it is a good practice to use a Child theme if you going to customise your website and add custom code to it.
It is not recommended to edit plugin core files, as your changes will be lost after each plugin update. For this reason, all your customisations should be added to your child theme files.