Mine were observant too, but they were Cooper's age in the early 2000's/ and the right height/weight to be sitting upright in forward facing seats at 22 months. Had a much better view and were not reclined, facing the back seat/straps at infant setting.
There's always risk with murder. I can't think of any murder case I've followed where the murderer had 100% assurance before doing it that it would all go off without a hitch or they wouldn't be caught in the act, interrupted, seen, etc. And if he has a serious personality disorder, the risk taking is part of the package.
As far as asking kids to 'pipe down'

, I'm reminded of a time where we were driving to the beach for vacation when our kids were 2 and 4. My husband was expecting a phone interview for a big promotion while we were driving so we made sure I took that driving shift, and we told the kids "When Daddy's phone rings, we have to be VERY silent for the WHOLE phone call. Daddy will be talking to an important person for a new job."
They did it. They were silent for 45 minutes. Not a peep, whine, or poke at their sibling. And I think they got extra play time at McDonald's as a reward.
As you mentioned earlier, perhaps Cooper was used to his needs being ignored when with his father. Perhaps it would take a lot to make him bother to cry for attention. Just don't know what their dynamic is, or if Cooper was used to neglectful parenting from his dad.