iPad app for movies or tv shows without wifi

Hello I have an iPad mini and I want to turn it into something for my son for in the car to watch a movie or a show because he hates car rides. I wAs curious if any mom's out there do this? And maybe what apps I could purchase or get for free that can do this? Obviously the iPad wouldn't be connected to wifi in the car so an app that can play movies or shows without wifi. Thanks!