I have some doubts that it's a person. The head seems to be the wrong size and shape. The mouth looks wrong. But it could be a kid back there photobombing.
I mean, I guess it doesn't really matter. And as you say, pareidolia is a powerful force. I'll just put this out there, though, to say why I think it's a kid:
1. A kid is a thing likely to be in the back seat, and can lean to the side in a way that would make them visible in the picture. So at the very least it's possible.
2. Skin tone looks right, assuming there's a tinted window in the back. That certainly appears to be the case.
3. Based on my experience of having lots of kids in my back seat of varying ages, the height of the head looks fine.
4. Assuming it's a kid's head, we would expect to see hair on top and a mouth and chin on the bottom. Obviously the eyes and nose are obscured. There do seem to be those things, and the lips appear to be slightly redder than the surrounding skin as you would expect (again, account for the tinted light from the window).
5. Look just under the head, it looks like the fabric of the shirt has a bump where the collar would be. So, you know, could be a collar.
6. Look at the shoulder. Now look at the other picture, without the kid. Note that you can still see a bit of the green shirt. Now look at the angle, it's much more level. This is consistent with the kid leaning to the side in the one picture but sitting up straight in the other.
So that's my deal. But okay, I do totally concede that I don't know for sure and it could always be photoshop or some strange thing that happens to look like a kid from the right angle. I just think that "it's a kid" is the most plausible answer.