We use portable DVDs on long trips ( more than 3 hours or so). One is a battery powered off brand unit that came with headrest attachment straps for about $100 or so. The other is a 2-screen Car DC powered unit that came with headrest attachment straps and a game unit (
http://www.walmart.com/catalog/product.do?product_id=4100513 ). They all have inputs and outputs so you can share programs or plug in a game unit.
Unlike the 9" and bigger built-in screens that drop from the roof, these don't obstruct the driver's visibility at all since they are on the back of a head rest. They also aren't visible much, if at all, to other drivers. Since they are mounted behind headrests, it's hard to see how they would pose a significant risk in a frontal crash. I suppose the attachment straps could break in a severe side or rear crash. It's also something a child's head could hit if they are thrown that far forward. I don't consider those to be anything but very unlikely risks, but others might be concerned.
I see off-brand models with 7" and smaller screens for $50 sometimes. The more reputable brands like Sony and Toshiba usually cost a little more. Not all of them come with straps to mount them in a vehicle, though. Granted, they're so small, they can be used hand held. Probably no more dangerous on a child's lap than a large book or a Gameboy.