what could the teacher do?
If you know the child, you know the parent.
I'd say something to the parent. If not brave enough to say it straight, maybe say "oh I saw you the other day, X in the front seat with no seatbelt, aren't they monkeys? mine are forever taking theirs off too! Scares me witless when they do it."
or get into a conversation and start talking about this poor kid who got thrown through a windscreen cos some idiot hit the car but he wasn't wearing a seatbelt.
cos people will say oh it's not your business, but this whole never say anything to anyone thing we have going on - is it really good? supposing that child DOES get thrown through the windscreen - do the never interfere brigade think the response should be "oh well, i knew but it wasnt really my place to say anything, never mind, eh?" or "its a pity, but it really wasn't my problem"