I'd recommend at least a 7" screen and 2 screens.
Something like this one from Netto should be fine.
We have a cheap one from Argos I think and it's great for long motorway journeys. Our dd's are 7 and 4 now so they have earphones too, the know the old fashioned sort that you used to have with your Walkman?
Just make sure it comes with all the necessary adaptors for the car and most are fine.
You can spend hundreds of £ btw