I'll assume you're referring to Dinosaur National Monument. The Cleveland-Lloyd Quarry is a much smaller scale, low key attraction. Very nice, but check to make sure it has reopened. There is remodelling in process, which will presumably upgrade the facilities.
Be aware that driving a significant distance on unpaved roads is necessary. And it will be very hot and dusty in July.
Dinosaur NM is a substantial distance out of your way, but the dinosaur quarry is quite impressive. There is the added attraction of the Green River, a magnificent desert watercourse.
It's 175 miles and 3 hours east from SLC to Vernal.
Driving an easy road. From Vernal there are two entrances to Dinosaur NM. One from Vernal one from the Colorado side of the UT/CO border. Closest is from Vernal.
If you really are interested in dinosaurs, this is the place to be.
Visiting the NM and driving south gives you the opportunity to see Canyonlands and Arches NP near Moab. It will give you and your grandson and unforgettable impression.
If you want to know more about the natural wonders south of Moab.
How long will you be staying?
If you're interested in dinosaurs I'd make a stop at Thanksgiving Point 20-30 minutes south of Salt Lake City to see the North American Museum of Ancient Life. Considering that it is one of the largest dinosaur collections in the world with 60 HUGE free-standing dinosaurs (think about walking directly underneath a Supersaurus, a Brachiosaurus, and two fighting T-Rexes!) and more than 800 smaller specimens it is not to be missed. I've been to the Field Museum in Chicago which is world-famous for its palentology displays but it cannot even compare to what you'll see in the Utah museum. It's also better than the other Utah dinosaur sites that you mentioned, in my opinion. Check out their website at www.thanksgivingpoint.com/museum/index.html
but also google "north american museum ancient life" to read some news items about it as well.
Thanks to all of you who responded to my questions. Tet14 - we will be in Utah for 11 days altogether. Really looking forward to the trip.
For I nice and relatively quiet itinerary I suggest:
Day 1: Arriving SLC
Day 2: SLC-Vernal (175 M)
Day 3: Visit Dinosaur NM
Day 4: From Vernal to Moab (210 M)
Day 5: Visit Arches and Canyonlands
Day 6: From Moab to Monument Valley. Visit
Natural Bridges NM, Gooseneck State
Park, Moki Dugway, Mexican Hat. (175 M)
Day 7: Visit Monument Valley in the morning
drive to Chinle. (150 M).
Day 8: Visit Canyon de Chelly.
Day 9: From Chinle to Show Low (200 M) visit
Hubbell Trading Post, Painted Desert
and Petrified Forrest on your way.
Day 10:From Show Low to Phoenix (175 M)
Day 11:Phoenix to SA.
Make reservations as soon as possible because you're in a busy part of the year.
Buy a National Parks Pass. Saves you money.
Thanks tet14 for the information. We have airline tickets and car, just trying to decide on where to stay. We have a Golden Age (or something like that) for the NP. Grammy