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
}