We have a cheapish DVD player with twin screens which attach to the back of the headrests and for the last 2 years they have served their purpose. But my mate uses an Ipad for each child in conjunction with a wireless hard drive. This way the kids are able to watch different movies at the same time. So when our DVD player dies, I will be going out to buy a wireless hard drive especially considering we already have a couple of Ipads.
But whatever way you go, just make sure you get them headphones otherwise it means while the kids are watching a movie, it hard for the parents to talk or even listen to the radio.