SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(602, 603, 604, 605, 606, 607) 
  AND category_id != 264

Query time 0.00017

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.009357655,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_categories",
          "access_type": "range",
          "possible_keys": ["PRIMARY", "parent", "p_category_id"],
          "key": "parent",
          "key_length": "3",
          "used_key_parts": ["parent_id"],
          "loops": 1,
          "rows": 19,
          "cost": 0.009357655,
          "filtered": 100,
          "attached_condition": "cscart_categories.parent_id in (602,603,604,605,606,607) and cscart_categories.category_id <> 264",
          "using_index": true
        }
      }
    ]
  }
}

Result

category_id parent_id
608 602
609 602
610 602
611 603
612 603
613 603
614 604
615 604
616 604
617 605
618 605
619 605
620 606
621 606
622 606
623 607
624 607
625 607
626 607