<?php
 
$array = [
	'd' => [
		'f',
		'e',
	],
	'a' => [
		'c',
		'j',
	],
];

function recur_natsort(&$array) {
   foreach ($array as &$value) {
      if (is_array($value)) recur_natsort($value);
   }
   return natsort($array);
}

array_multisort(array_keys($array), SORT_NATURAL, $array);

recur_natsort($array);
 
var_dump($array);