How to add styles to a submenu page?

One option is to examine the $hook and see, if it matches the parent or any of the child menu page slugs.

function load_entry_scripts($hook)
{
    $hook_parts = explode('_page_', $hook);
    $menu_slug = array_pop($hook_parts);
    
    if ( ! in_array( $menu_slug, array('my-parent-page-slug', 'my-child-page-slug') ) ) {
        return;
    }

  // add styles && scripts
}