Streaming video equates to 1gb of data per one hour of video. This is why campgrounds restrict streaming as they have just so much bandwidth to share with everyone staying and using it. If you want to purchase the data it is going to cost a great deal. There are all types of plans available - Verizon is currently doing deals but that still is hundreds per month - and if Verizon service is spotty this will not help.
Since they are streaming movies, TV shows and music they certainly could get this with a Dish TV or Direct TV connection and just watch the shows and movies as they are broadcast for a similar cost that it would be at home. Nothing is cheap anymore. You can rent DVDs from Redbox machines for $1.20 and they have locations all over. Then they can use the campground wifi for basic browsing and email.