SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(611, 612, 613, 614, 615) 
  AND category_id != 264

Query time 0.00015

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_categories range PRIMARY,parent,p_category_id parent 3 5 Using index condition; Using where