Display same menu for parent and all its children

Yes, create 3 custom menus and use a conditional.

Example you have 3 custom menus named, Products, Services,Training.

Open the section of the page you want to insert code in the header.php or where ever you need it. file and use:

if (is_page( array( 42, 'Products' ) )) {
  wp_nav_menu('menu' => 'Products');
}elseif(is_page( array( 2, 'Services' ) )) {
  wp_nav_menu('menu' => 'Services');
}elseif(is_page( array( 42, 'Training' ) )) {
  wp_nav_menu('menu' => 'Training');
}else {

}