I have three kiddos and opted for the wifi and Macally iPad holder and then bluetooth it through the system - I don't mind listening to it, at least I know what they're watching. Plus, now they can watch Netflix, HBO Go, Prime, etc. I could still rip DVDs and put them on the iPad. Fortunately they're all around the same age so not too much of battle (6, 8, 9). Hope your kiddos like your setup - it looks sweet!
there is one 12V outlet in the trunk, otherwise you can not plug these tablets in to the USB. You would have to install the dvd players into the seats power source. There is a couple of different manufacturers besides Xtrons, but pure bliss having two kids with their own headphones watching the movie they want.