As long as the advert isn't showing the stuff that got the game the 18 rating, why the bother? You get trailers for 15 and 18 rated films on during the day, during family viewing. It's a non-issue, or should be unless you want to be like those reactionary parent groups in America that are constantly complaining about anything that doesn't fit into an idealistic 1950s 'gee whizz' world.
Presumably the OP is suggesting they shouldn't be allowed to advertise things that children aren't allowed to buy during televised (afternoon) football matches.
So that would be things like betting websites, alcoholic drinks, cars, etc... ie pretty much 80% of all football advertising/sponsorship then.