A couple of things come to mind. One is that, today, many children in families where both parents are Catholic are choosing not to remain Catholic. Which begs the question: How much is this about parental influence and how much is about societal and generational influence? Millennials are less inclined to be "joiners" anyway, whether you are talking about churches, political parties, civic clubs, etc. 

Also, parents pass along to their children the values that are most important to them. There was a time when Catholics were less inclined to marry non-Catholics precisely because it was perceived as a potential problem in raising children. Those for whom raising their children Catholic were more diligent about dating only within the faith. It probably is still that way, the difference being that there are just fewer of those people today.