Get category children
Because the descendantOf function expects a parameter of type ElementInterface or an integer

 /**
 * Sets the [[descendantOf]] property.
 *
 * @param int|ElementInterface|null $value The property value
 *
 * @return static self reference
 */
public function descendantOf($value);


You'll have to insert the ID of your accessories element or the element itself

{% accessories = craft.categories.title('accessories').first() %}
{% set categoryGroup = craft.categories.group('productTypes').descendantOf(accessories) %}