What is the rental price per day of the most expensive children's film?
children's film refers to name = 'Children'; average price per day of most expensive film = Max(Divide(rental_rate, rental_duration))
SELECT T1.rental_rate FROM film AS T1 INNER JOIN film_category AS T2 ON T1.film_id = T2.film_id INNER JOIN category AS T3 ON T2.category_id = T3.category_id WHERE T3.name = 'Children' ORDER BY T1.rental_rate / T1.rental_duration DESC LIMIT 1