InnerBlock

Interesting attributes : allowedBlocks, template, InnerBlocks.

Example of a devblock/render.php

<?php

$classes_container = array( 'block__container' );
$classes_innerblock = array( 'block__innerblock' );

if ( !empty( $block['className'] ) ){
	$classes_root = array_merge( $classes_root, explode( ' ', $block['className'] ) );
}

$allowed_blocks = array( 'namespace/blockname' );

$template = array(
	array(
		'namespace/blockname',
		array(
			'className'		=> 'some-class'
		)
	)
);

?>

<div class="<?php echo join( ' ', $classes_container ); ?>">
	<InnerBlocks
		class="<?php echo join( ' ', $classes_innerblock ); ?>"
		allowedBlocks="<?php echo esc_attr( wp_json_encode( $allowed_blocks ) ); ?>"
		template="<?php echo esc_attr( wp_json_encode( $template ) ); ?>" />

</div>