In children's movies, which was rented the most?
children is the name of the category; rented the most refers to MAX(COUNT(title))
SELECT T.title FROM ( SELECT T4.title, COUNT(T4.title) AS num FROM customer AS T1 INNER JOIN rental AS T2 ON T1.customer_id = T2.customer_id INNER JOIN inventory AS T3 ON T2.inventory_id = T3.inventory_id INNER JOIN film AS T4 ON T3.film_id = T4.film_id INNER JOIN film_category AS T5 ON T4.film_id = T5.film_id INNER JOIN category AS T6 ON T5.category_id = T6.category_id WHERE T6.name = 'Children' GROUP BY T4.title ) AS T ORDER BY T.num DESC LIMIT 1