Another option for you if you happen to have iPad's or Android tablets in the house is something like what we do. We've got a Seagate wireless hard drive loaded up with a ton of movies for the kids as well as the big people. The HDD broadcasts on a wifi signal and your connect your electronic device to that personal wifi network and stream movies from the HDD to your iPad, tablet, laptop, phone, etc. The only challenge here is getting ripped copies of movies to save to the HDD. In the same vein if you buy movies that include a digital copy you can download the digital copy of the movies to iPad or tablet and view that way without needing the HDD. The only limitation here would be storage capacity on the device in question.
So the kids don't have to hold the device the entire time we've got the iPad's mounted to the headrests with a Ram-Mount handlebar iPad mount, sorta similar to this but with a different clamp.
This what is looks like all set up in the Jeep.
or
Quadratec doesn't sell it anymore. Why not just get a portable dvd player and either have her hold it or tie it to the back of the seat for her.
Exactly, you can get a portable DVD place with a headrest case for dirt cheap.