I agree that there should not be an issue if the KLM flight arrives on time. However, KLM schedules their flights more tightly than Delta, and as a consequence their arrival is more often late than Delta, particularly on long flights where headwinds, etc. cause flight times that may be 30-50 min longer than planned. I know that KLM's attitude toward any question for change is that "it is not possible", but if your parents do not want to overnight in SLC, my suggestion would be to lean on KLM more and get them on the AMS-SLC DL flight that is much earlier in the day, and then they can be booked on an earlier flight to JAC as well.