block: innerBlocks

index.js:

import { InnerBlocks } from '@wordpress/block-editor';

registerBlockType( metadata.name, {
	edit: Edit,
	save: props => { return <InnerBlocks.Content /> }
):

render.php:

<?php echo $content; ?>

in edit.js:

return (  <InnerBlocks { ...blockProps } /> )

Source: https://learn.wordpress.org/lesson/work-with-the-innerblocks-component/