Two ways to enqueue an asset on an admin sub-page.

enqueue-asset-on-admin-subpage.php

/**
 * You can hook directly into the `load-{page}` hook for a sub-page with
 * an action like this.
 */
add_action( 'load-myplugin-parent_page_myplugin-subpage', 'enqueue_myplugin_assets' );

/**
 * Or you can hook into `admin_enqueue_scripts` and check the current
 * screen to see if we're on the right page.
 */
add_action( 'admin_enqueue_scripts', 'enqueue_myplugin_assets' );
function enqueue_myplugin_assets() {

	$screen = get_current_screen();
	if ( !empty( $screen ) && $screen->base == 'myplugin-parent_page_myplugin-subpage' ) {
		// enqueue your assets
	}

}