SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    18348, 18320, 18349, 18323, 18333, 18347, 
    18350, 18360, 18367, 18368, 18369, 
    18370, 18371, 18372, 18379, 18380, 
    18394, 18397, 18398, 18404, 18405, 
    18406, 18407, 18408
  ) 
  AND category_id != 1

Query time 0.00005

JSON explain

{
  "query_block": {
    "select_id": 1,
    "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"],
          "rows": 48,
          "filtered": 100,
          "index_condition": "cscart_categories.parent_id in (18348,18320,18349,18323,18333,18347,18350,18360,18367,18368,18369,18370,18371,18372,18379,18380,18394,18397,18398,18404,18405,18406,18407,18408)",
          "attached_condition": "cscart_categories.category_id <> 1"
        }
      }
    ]
  }
}