Why isn't the cart.php module displayed on the OpenCart site?

Question

PHP

Good afternoon!
There is a site on OpenCart. In module /catalog/controller/module/header.php added

$this->children = array(
  'module/language',
  'module/currency',
  'module/cart'
);

but the file /catalog/controller/module/cart.php does not work in any way, the template is not displayed. In this case, there are no errors.
There is a nuance - everything worked, before I rearranged a couple of lines in places in header.php. Only returning to the initial state did not help to return the module. What can be wrong?
UPD: the $this->children = Array( ... ) construct did not write the new array into a protected variable of the inherited class.
The line $this->children[] = 'module/cart';
corrected the situation. But the question remains, why is that?