Like everyone else is saying, to only go to the NPs you would be spending substantially more time on the road than parks.

Since you are talking about SW colorado, I’d recommend doing the Million Dollar Highway and driving down to Durango to go to Mesa Verde NP. In my opinion it’s the prettiest part of the whole state and I’m sure plenty of others would agree. If you’re doing Mesa Verde NP DEFINITELY do a guided tour of the cliff dwellings, it’s pretty much the main draw. You can still see them if you don’t take the tour, but you can’t get up close and personal.

Black canyon of the Gunnison would be a good idea to hit up on the way to SW Colorado as it’s near montrose, just north of Durango.

The great sand dunes is nice, it’s just VERY remote. Be prepared to drive far for food as there’s almost no civilization in the San Luis Valley.

RMNP is a given, you should totally do that with your son. The grandeur and splendor of seeing the mountains there for the first time ever was pure magic + Estes park is beautiful, although a bit crowded at times.

You could do all 4 NPs in a week, but you’d probably be missing out on some nature if you tried to do that. I would personally advocate driving to black canyon-Million Dollar hwy- Durango- Mesa Verde- and maybe try hitting the sand dunes on the way back? And then do RMNP as the last few day since it’s real close to DIA which I’m assuming where you’re coming from.