I struggle with this because on the one hand, that kid might be in a really awful situation, and on the other hand, CPS may very well put them in a worse one.

This would probably just be a note in a file if you called (if that, since you don't know them. I guess you could give license plate number?), but if others report small instances it could accumulate to be enough for them to check on the kids.

I think you were right not to say anything in the moment. You're not sure of what you saw and you could have made the situation worse.

Honestly as upsetting as it is I probably wouldn't do anything if you're not sure of what you saw, but if it happens again or if you really feel compelled to do something I can't think of anything to do other than calling CPS.