Database: movie_3
Question: What is the rental price per day of the most expensive children's film?
SQL: 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