SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    294, 296, 295, 297, 298, 299, 300, 301, 
    302, 303, 304, 305, 306
  ) 
  AND category_id != 264

Query time 0.00026

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "23.28"
    },
    "table": {
      "table_name": "cscart_categories",
      "access_type": "ALL",
      "possible_keys": [
        "PRIMARY",
        "parent",
        "p_category_id"
      ],
      "rows_examined_per_scan": 98,
      "rows_produced_per_join": 12,
      "filtered": "13.27",
      "cost_info": {
        "read_cost": "20.68",
        "eval_cost": "2.60",
        "prefix_cost": "23.28",
        "data_read_per_join": "43K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ],
      "attached_condition": "((`danishecarter_latest`.`cscart_categories`.`parent_id` in (294,296,295,297,298,299,300,301,302,303,304,305,306)) and (`danishecarter_latest`.`cscart_categories`.`category_id` <> 264))"
    }
  }
}