You can filter template_include and replace the single-community.php with a single-child-community.php.
Example
add_filter( 'template_include', function( $template ) {
if ( ! is_singular() )
return $template; // not single
if ( 'communities' !== get_post_type() )
return $template; // wrong post type
if ( 0 === get_post()->post_parent )
return $template; // not a child
return locate_template( 'single-child-community.php' );
});