SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    21092, 21093, 21094, 21095, 21096, 21097, 
    21098, 21103, 21104, 21109, 21110, 
    21111, 21112
  ) 
  AND category_id != 1

Query time 0.00006

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": 21,
          "filtered": 100,
          "index_condition": "cscart_categories.parent_id in (21092,21093,21094,21095,21096,21097,21098,21103,21104,21109,21110,21111,21112)",
          "attached_condition": "cscart_categories.category_id <> 1"
        }
      }
    ]
  }
}