The comment about my son's drawings are admittedly "color commentary." I'd say the meat of my argument is in the first statement you didn't quote: "of course, there is no other plausible explanation for these images except that the Indians were familiar with living dinosaurs."
The central issue, I believe, is whether or not direct encounter with living dinosaurs is the only plausible explanation for the pictographs. I think not. Just off the top of my head, I can think of several. The possibility that Indians extrapolated from fossils encountered by themselves or others during their many years of close familiarity with the American landscape. The possibility that Indians participated in cross-cultural exchange of stories that became mythological raw material during contacts with travellers from other civilizations, such as African or Oriental or European ocean travellers with extensive mythological beastiaries of their own. The possibility that Indian shamans experienced during the altered state of consciousness of a dream or trance images that were later portrayed as mythical creatures that conceivably resemble dinosaurs. Even the possibility that the petroglyphs are fakes.
Not saying that I prefer one or another of these other possibilities. But it's just silly to say that the only plausible explanation is encounter with living dinosaurs. As silly as my pointing at my son's drawings as proof of space monsters. Why should the "living encounter" hypothesis be preferred over the ones I listed? No reason I can think of, unless one is trying to make the petroglyphs support a particular theory of the age of the earth or the age of living creatures.
Hmmmmm....I wonder which one of these possibilites best meets the requirements of Occam's Razor?