Factory installed units are usually 7" TFT active matrix LCD with 16:9 ratio.
Be aware that many cheaper portable DVD players are using STN LCD instead of TFT. STN in sub par in contrast to TFT.
Panasonic, Pioneer, Sony, Toshiba, they all use TFT. Polaroid uses lower end TFT. Mintek and the like of them use STN.
The bad thing of using splitter is that you can't control each headphone separately.
The best thing to do is to use splitter, then feed each output to a Boostaroo (can be obtained at any Radio Shack) so each headphone can control their own volume control
Last but not least, do NOT get cheap headphones, they tend to peak at higher frequencies thus forcing the user to crank up the volume in order to hear the dialog. At the same, this will over emphasize the higher frequencies and will result in ear fatigue, or worse, hearing damage.
Rule of thumb, if you spend less than $20 on a headset, you're looking for trouble in the long run.