SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(503, 504, 505, 506, 509, 853)

Query time 0.00018

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_categories range parent parent 3 6 Using index condition