Function to return true if current page has child pages

Your above function tests whether a page is a child page of some other page, not whether it has children.

You can test for children of the current page like so:

function has_children() {
    global $post;

    $children = get_pages( array( 'child_of' => $post->ID ) );
    if( count( $children ) == 0 ) {
        return false;
    } else {
        return true;
    }
}

Further reading:

• wp codex: get_pages
• php manual: count