Add order meta to child orders in WC Marketplace

functions.php

<?php

add_action( 'wcmp_checkout_update_order_meta', function( $vendor_order_id, $args ){

    $parent_order = wc_get_order( $args['order_id'] );

    $child_order = wc_get_order( $vendor_order_id );

    foreach( $parent_order->get_meta_data() as $meta ){
        $child_order->update_meta_data( $meta->key, $meta->value );
    }

    $child_order->save();

}, 11, 2 );