Definitely 287 to Amarillo and I-40 for speed.
Never been a fan of Cadillac Ranch, but if you want the kids to do some random vandalism, fine. It's on the south side of I-40, so you'll have to loop around to get to it.
Second Russel's Truck Stop. Always nice, and the free museum will accept donations, which are used to feed homeless in Albuquerque, so keep that in mind.
Not a lot of good, quick, stops along the way. And I'd eat lunch in Amarillo, not going to be another decent place until Santa Rosa. Tyler's BBQ is not far off I-40 and top notch. Blue Sky Hamburgers are right off I-40. Some better places out there, but for a fast stop with kids, I'd put those at the top of the list.
Charles Goodnight home in Goodnight is a good stop if you're a Lonesome Dove fan. Blue Hole in Santa Rosa isn't far off the Intestate, and interesting, but not much to do if you're not swimming. The old Route 66 alignment in Tucumcari is as well preserved as you'll ever see.
The Medical Center area in Amarillo has some nice walking paths, a splash pad, and the Discovery Center.
Estelline is famous for being a speed trap.
National Nuclear Museum is high quality. Not sure a 5 and 7 will get much out of it, but they might enjoy the planes and missiles in the back yard. The natural history museum has some dino stuff, as I recall, and the Zoo./Botanical Gardens are good.
If you want more ABQ suggests, reply or PM. Lots to do there, unless you are going further.
Always take everything out of the car at the motel, and hats and sunscreen, because the sun at that altitude (5,280) is brutal