(WordPress) [wpb_childpages]

(See https://stackoverflow.com/questions/49183161/show-child-pages-of-specific-parent-on-any-page-in-wordpress/ for details.)

STEP 1: Add these to your theme's functions.php file:
--

function wpb_list_child_pages( $atts = array() ) {
	$atts = shortcode_atts( array(
		'id'          => 0,  // The parent post's ID. Set this or the slug.
		'slug'        => '', // The parent post's slug. Set this or the ID.
		'menu_toggle' => 0,  // Enable menu's toggle link? 1 = yes; 0 = no.
		'show_menu'   => 1,  // Show the menu by default? 1 = yes; 0 = no.

		// By default, the toggle link's label is "Toggle Menu"; if you want a
		// different label, set the menu_toggle to the label you prefer. E.g.
		// [wpb_childpages slug="parent-page" menu_toggle="Show/Hide Menu"]

		// If menu_toggle is disabled (i.e. 0), the show_menu is "ignored".
	), $atts );

	if ( $atts['slug'] ) {
		global $wpdb;

		$q = $wpdb->prepare( "SELECT ID FROM {$wpdb->posts} WHERE post_name = %s", $atts['slug'] );
		$post_id = $wpdb->get_var( $q );

		if ( ! $post_id ) {
			return '';
		}
	} else {
		$post_id = absint( $atts['id'] );
	}

	$post = get_post( $post_id ); // WP_Post on success.
	$childpages = '';

	if ( $post && is_post_type_hierarchical( $post->post_type ) ) {
		$childpages = wp_list_pages( array(
			'child_of' => $post->ID,
			'title_li' => '',
			'echo'     => 0,
		) );
	}

	$html = '';
	if ( $childpages ) {
		// This class is required for the JS functionality.
		$class = 'wpb-child-pages';

		$menu_style = '';
		if ( $atts['menu_toggle'] && $atts['show_menu'] ) {
			$class .= ' menu-shown';
		} elseif ( $atts['menu_toggle'] ) {
			$class .= ' menu-hidden';
			$menu_style = 'display: none;';
		}

		$html = '<div class="' . $class . '">';

		if ( $atts['menu_toggle'] ) {
			$label = ( '1' != $atts['menu_toggle'] ) ?
				$atts['menu_toggle'] : 'Toggle Menu';

			// Add the link for showing or hiding the menu.
			$html .= '<a href="#" class="toggle-menu">' . $label . '</a>';
		}

		// Add the menu.
		$menu_style = $menu_style ? ' style="' . $menu_style . '"' : '';
		$html .= '<ul' . $menu_style . '>' . $childpages . '</ul>';

		$html .= '</div>';

		static $js_added = false;
		// Add the inline JS code which handles the toggling stuff.
		if ( ! $js_added && $atts['menu_toggle'] && wp_script_is( 'jquery' ) ) {
			add_action( 'wp_print_footer_scripts', 'wpb_list_child_pages_script' );
			$js_added = true;
		}
	}

	return $html;
}
add_shortcode( 'wpb_childpages', 'wpb_list_child_pages' );

function wpb_list_child_pages_script() {
	?>
<script>
jQuery( function( $ ){
	$( '.wpb-child-pages' ).on( 'click', '.toggle-menu', function( e ){
		e.preventDefault();

		// Find the menu.
		var $wrapr = $( this ).closest( '.wpb-child-pages' ),
			$menu = $wrapr.children( 'ul' );

		// Toggle the menu.
		if ( $menu.is( ':visible' ) ) {
			$menu.slideUp();
			$wrapr.removeClass( 'menu-shown' );
		} else {
			$menu.slideDown();
			$wrapr.addClass( 'menu-shown' );
		}
	} );
} );
</script>
	<?php
}

STEP 2: Usage examples:
--

1. Use the parent post's ID or slug:

	[wpb_childpages id="123"]
	[wpb_childpages slug="parent-page"]

2. The menu's toggle link is disabled *by default*; so to enable it, set the
   menu_toggle parameter to 1, like so:

	[wpb_childpages id="123" menu_toggle="1"]

3. The menu is shown/expanded by default; so to hide it by default, set the
   show_menu parameter to 0, like so:

	[wpb_childpages slug="parent-page" menu_toggle="1" show_menu="0"]

4. To use a custom label for the menu's toggle link, set the menu_toggle to
   the label that you prefer. Example:

	[wpb_childpages slug="parent-page" menu_toggle="Show/Hide Menu"]

Note that show_menu is "ignored" if the menu_toggle is 0 (or empty).