You'll probably find that the toddler will prefer that. After all, he is not going to remember anything much about the trip. His grandparents will likely spoil him.

If this were me, I would choose UT and CO, leaving the northern parks for another trip.... when the toddler is older. There is so much in UT and CO that you could spend a month there and only scratch the surface. Do you have some good maps of those States. That will show you just how many NPs and other attractions there are. There are also many public lands camping spots scattered throughout those States.

Lifey