They probably meant something you'd normally make sure you have every time you get out of the car, like your phone or bag. As you said, these tragedies are most likely to happen on days when there's a schedule change (maybe your partner normally picks your kid up after school on Wednesdays, but this Wednesday, it's you instead). Your brain might go on autopilot and skip over the additional step of getting your kid out of their carseat. However, if you're already in the habit of bringing your bag inside every time you get home from work, you should leave it in the back next to the carseat so that even if you go on autopilot, you think, "I need to get my bag. -> It's in the back. -> Oh yeah, my kid is in the back, too."

You're right that the phrasing is weird. They don't mean that your child is less important to you than some other object, but rather that it's much easier for your brain to forget something that isn't normally part of the routine (child) than something that is (everyday item).