We have a Phillips brand 9" split screen portable DVD player. It's perfect for long car rides, because we have our older two facing forward, and our youngest still rear facing. So this way, we can strap one screen on the back of the passenger headrest and the other one on the front of the seat my youngest is sitting in, and the three kids can all watch the same thing. Plus, then they aren't holding it and fighting over it, and risking one getting broken. It came with the straps to mount the screens onto the seats. But we didn't pay that much for ours. I'll have to ask hubby where he got ours at. I think it must have been a sale, and I seem to remember it was at Best Buy, or someplace like that. It might have been a great Black Friday deal, but we got it several years ago.
I don't know how well an IPad would work, but this would be more versatile. You'd have to figure out a system to mount it so that they wouldn't fight over it. But then you would have the options for apps and lots of other things too. And if kiddos fall asleep, then mama could use it too.
The downside is that you'd have to buy the movies and shows to watch on it, instead of getting to use the DVDs you already own.
ETA: I just saw someone post a way to mount the IPad. If they are both forward facing, I would go this route. We never use the portable DVD players outside of trips, but the IPad or any of the other pad devices could still be used at other times.