SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(270) 
  AND category_id != 264

Query time 0.00011

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.001792605,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_categories",
          "access_type": "ref",
          "possible_keys": ["PRIMARY", "parent", "p_category_id"],
          "key": "parent",
          "key_length": "3",
          "used_key_parts": ["parent_id"],
          "ref": ["const"],
          "loops": 1,
          "rows": 1,
          "cost": 0.001792605,
          "filtered": 100,
          "attached_condition": "cscart_categories.category_id <> 264",
          "using_index": true
        }
      }
    ]
  }
}