There is a theory going around that parents (or grandparents) should just let their children do whatever and never criticise their decisions even when they are convinced that these decisions are wrong, because "you must let your children follow their dreams".
Some of the more ardent proponents of this theory would tell you, for instance, that you can't forbid a kid of say 13 drink alcohol, or have s8x even though the law (!) still says it's forbidden and the parents or legal guardians are supposed to enforce it.
The majority wouldn't probably go that far but they fully agree that when your child becomes a legal adult you should just shut up and never try to correct his behaviour in any way. That's simply ridiculous. Your child (or even a grandchild) is your flesh and blood, the product of the generations of ancestors and you as parents have sacrificed a lot and have a huge investment in his spiritual, emotional and material well-being.
When they just hit 18, they mostly still live at home (at least, over here) and are unable to support themselves, and as long as they are subsidised by their parents, those parents absolutely have a right to interfere into their life.
Of course, when your child is say, 30, and lives separately, you aren't responsible for his actions any more and your influence is very limited, but still if he or she chooses a wrong path, isn't it your Christian duty to at least try and talk to him? I mean, people would go out and preach to drug addicts or pr*stitutes, who are total strangers, isn't it only logical to do the same for your own child?
Those who tell you that you should just cut your children loose and always support their choices even when you find them morally repugnant, don't have either yours or your children's best interests in mind.
P.S. I am very well aware that while some parents are too permissive, others would go so far as to even try to ruin their children's marriages for petty reasons. I don't mean to say it's OK, but if you see your grown up child falling into sin isn't it only natural and logical to try and warn him? Even if he doesn't listen, you can comfort yourself with the thought that you've done what you could.
As for marriages, it's my opinion that the parents should absolutely voice their thoughts about their children's choice of the spouse but once married, even if you disagree with the choice, you should leave them alone since divorce is a really really huge sin. You don't want to have THAT on your conscience.