SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    3826, 3827, 3828, 3829, 3830, 3831, 3832, 
    3833
  )

Query time 0.00017

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_categories",
      "access_type": "range",
      "possible_keys": ["parent"],
      "key": "parent",
      "key_length": "3",
      "used_key_parts": ["parent_id"],
      "rows": 8,
      "filtered": 100,
      "index_condition": "cscart_categories.parent_id in (3826,3827,3828,3829,3830,3831,3832,3833)"
    }
  }
}