SELECT 
  `product_id` 
FROM 
  cscart_products 
WHERE 
  `parent_product_id` = '33721'

Query time 0.00053

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_products",
          "access_type": "ref",
          "possible_keys": ["idx_parent_product_id"],
          "key": "idx_parent_product_id",
          "key_length": "4",
          "used_key_parts": ["parent_product_id"],
          "ref": ["const"],
          "rows": 15,
          "filtered": 100
        }
      }
    ]
  }
}