Oh, you are gonna love Colorado for dinosaurs. We did a dinosaur trip with our 3 grands. There are good dinosaur exhibits right around Denver. Our favorite was at Woodlands, where there is a museum and an active laboratory for extracting the bones from the stone. This is about 20 miles outside Colorado Springs. Then there is the AF academy to drive around in CO Springs.
Then right on the outskirts of Denver there is Dinosaur Ridge and a small museum (near Morrison) and finally in Morrison there is another museum and exhibit. And in the front yard of that museum is a big boulder that kids can pound on to see bones (or at least there was one). And finally at the Denver Museum of Natural History is a wonderful exhibit.
There is a book called Denver's Oceans (available at one of those museums) that describes the era when Denver was literally underwater before the Rockies were formed and the dinosaurs roamed--and left their footprints that you can see alongside the road at Dinosaur Ridge.
I guess what I am saying is that you don't need to go any further west to see some wonderful exhibits and artifacts. Enjoy. We REALLY did.

I would add that this is quite a compact area from where you are in BV and it would be easy to spend one night in CO Springs/Woodlands and then your remaining time in Denver. Around the Tech Center on the southern edge of Denver are many hotels with very reasonable prices and that is near Morrison.
There is also the Denver zoo to enjoy and the Denver Art Museum has wonderful native American exhibits--lots to do and enjoy with a young-un!
Additionally there are volcanic remains to be seen along i25 around Castle Rock--just small little cones. AND don't forget to see the Red Rocks Amphitheater at Morrison!!

The fastest route from Buena Vista to Dinosaur National Monument (Visitor's Center) will take you just over 4.5 hours (261 miles).

The fastest route from the Monument to Denver will take you 5.5 hours (using the Visitor's Center as a starting point).

Dinosaur fossils are not visible in the Colorado portion of the monument - only on the Utah side.

More details here:
https://www.nps.gov/dino/planyourvisit/directions.htm

I think the info above is interesting and helpful. and we did not go to the Monument. I will say that the fossils visible along Dinosaur Ridge are VERY interesting and well explained with diagrams. And I will say that 6 year olds will get "caught up" (as in attention span) on looking for these (AND having parents try to explain graphically how the beasts made them) fairly quickly and will be more interested in the recreations of the various inhabitants of that age that are in the various museums. Just my opinion from doing it with an 8 year old and 2 6 year olds!!
One reason we did our trip was for the shortness of the car rides basing in Denver!!



https://www.google.com/search?rlz=1C1CHBD_enUS811US811&tbm=isch&sa=1&ei=LN0tXbj1I43btQbR0KfAAg&q=dinosaur+track+image+in+rock+Dinosaur+ridge&oq=dinosaur+track+image+in+rock+Dinosaur+ridge&gs_l=img.3...9671.13915..14245...0.0..0.68.853.15. .....0....1..gws-wiz-img.......35i39.RwWt7QlabsQ

If you can, the dinosaur quarry face at DNM is an incredible sight to see at any age.

I will emphasize that the places I gave you will be VERY satisfying for you and your child. We had a wonderful time. And do not overlook Denver itself in this.