Depending on the number of kids you are transporting (man that sounds insensitive), here is a cost-effective idea. I agree the DVD player in the roof is outdated but I hear ya on all the DVDs. If you can find the time, this is precisely what I would do.
Buy $50 Amazon fire tablets for each child. Get headphones to go with them. Buy the Western Digital MyPassport storage device. It has 3+ TB of storage. Burn all the DVDs in your collection (Mac user, use Handbrake, PC User tons of apps)and put them on the Passport. Now EACH child can watch what he/she wants out of your entire collection and you don't need to haul that DVD library. You then have happy kids with cheap tablets, happy spouse in the passenger seat, and the means to watch all of these videos anytime you want vis-a-vis wifi (planes, trains, automobiles and of course home). It can also simultaneously back up your data files.
You don't have to pay someone (or God help you, do it yourself) to install an aftermarket DVD player in your roof. Win-win-win.
Good luck on your project!