Need to override file included in parent theme

Hello you would have to edit the parent theme as there is no way to overwrite the include_once 'bookingtable/bookingtable.php';.

So to keep edits to the parent theme minimal, the whole code can be rewritten using wordpress get_template_part() as

if ( class_exists('rtbInit') ) {    
    get_template_part('bookingtable/bookingtable');
}

This way you can then copy the file to your child theme and make edits there.