Just a thought, but you may want to consider portable DVD players.
In my last SUV I was forced to buy the rear entertainment because it was the only way I could get NAV. We only let the kids use it on long trips, so it got used around 6 or 7 times. Not a very good investment!.
With the GX, we decided to skip the rear entertainment system and get a portable DVD player with a strap thingy that hooks onto the two front seats and suspends between them. At around $300 it was a great solution and saved a lot of money. Heck, you could buy 8 of them for the cost of your build in system! Also, they can be used outside the car. I guess if your kids are really little, and/or will watch a lot, the built in system would be easier to deal with.