A recent UK survey found that parents tend to let their children play adult-rated video games even though the parents know the games are 18-rated. Parents are aware of the games classification system, but tend to treat it as a guide rather than a definite prohibition. Apparently part of the problem is that parents don't seem to know much about the actual content of the games their children are playing. The obvious solution is to require parents to play more video games.
It's also not clear how the children are obtaining the adult-rated games. It could be that parents are buying them, again because they don't understand the significance of adult classifications. If the children are obtaining the games on their own, however, that becomes less a parenting issue and more of a game industry issue.