We recently traded in our truck for a minivan... the upside is that it came with an installed DVD system like
this oneThe Sony MV7101 - All-in-One Mobile Video System w/ DVD Player, Flip Down Monitor, & Wireless Headphones.
The sound is broadcast over the FM spectrum so you can play it on the regular audio system. This is the first one we've ever had or used. It's nice.
I have stopped a crying fit from a cranky 9 month old by putting on the Leap Frog "Learning Letters" DVD.
As for the trip... I am not so sure I'd want to drive the whole thing in one shot. When you factor in litle kids, the time will shoot up to something like 20 hours. Every stop (including the frequent potty breaks and legs stretches) will add up. Now the I have a little one, I am noticing that everthing takes just that much longer to do.
Another thing to consider is really fair to the little ones to be strapped into car seats for that long? If this is the first long trip, how it goes sets up their persecption of future family trips. Enjoy to adventure of the country that you cross. If possible check out one or two places along the way to break up the monotony... for all.
Do you plan to take the northern or southern route? The northern route through Butte, Montana is about 35 miles longer than down through Boise, Idaho. Google the towns either way to see if there things you'd like to stop and see.
Do you really want to pull into your inlaws driveway tired after nearly a 24 hour non-stop drive with a couple bored and restless little ones?
Hope that helps.