<?php

    global $post;

    function showSubpages( $post_id, $current_level ) {

    $subpages = get_posts( array(
        'post_type' =>'page',
        'posts_per_page' =>-1,
        'post_parent' => $post_id,
        'order_by' => 'title',
        'order' => 'ASC' ) );

    if ( empty($subpages) ) {
        return;
    }

    echo '<ul class="subpages level-'.$current_level.'-subpages">';

    foreach ($subpages as $page) {


        $image = get_the_post_thumbnail( $page->ID, 'thumbnail' );

        echo '<li>';

        echo '<a href="'.get_permalink($page->ID).'">';

        echo '<span>';
        echo apply_filters( 'the_title', $page->post_title );
        echo '</span>';

        echo '<div class="feature_image">' . $image . '</div>';
        echo '</a>';

        showSubpages( $page->ID, $current_level+1 );

            echo '</li>';

        }

    echo '</ul>';

    }

    showSubpages( $post->ID, 1 );