Before getting anywhere near that topic, it helps for parents to instill a proper understanding of self-control, different roles and responsibilities of one's state in life, and a proper time and place for everything. For example, getting things done vs. sleeping late and goofing off, moderation vs. gluttony, restraint in how and what one speaks vs. letting it rip on impulse. Basically that our bodies and natural faculties are for what God intends, not for our whims.
When these general standards are clearly taught early on, and the parents set a good example too, then talking about the well-ordered purposes of what's reserved for a married couple becomes a natural continuation of what's already learned.
This.
This was instinctual for me. It's holistic, it starts when they're 2. You teach them mortification, self-control, don't give in to their whims, don't stuff them with food during Mass, teach them to be busy/industrious and not laze around. For example, I *never* allow my kids to "roll around on the floor", not on vacation, not on Christmas, not on Sundays. By "roll around on the floor" I mean just be 100% idle, sometimes laying down, being silly, just 100% unstructured existing, doing and saying whatever pops into their head. I always tell them "Stop it, get up, go pick an actual game and play it". They could literally play ANY game, even a made up one -- just not devolve into drunk silliness and idleness. Again, it's not the laughter that's the problem -- is the physical aspect of completely letting go of all inhibition with regards to the body. That's a bad habit I don't permit.
Even major holidays, vacations, and Days of Rest should have *structure*. Time for prayer, daily chores, study, work (where appropriate) and RECREATION. But even recreation must have ACTUAL GAMES or activities. You don't just laze around, bored, waiting for something to happen. I'm a strong believer of the old adage, "An idle mind is the devil's workshop"
One of the things Fr. Doran (ex-SSPX, now a Conciliar priest in the Maronite Rite, the most liberal of Eastern Rites) got right was the thing about kids eating during Mass. He said that the 2 year old eating Cheerios during Mass "to keep him quiet" today will be having trouble with purity, even fornication, in 15 years. It made sense to me. It's the same mental pathway: Give in to your body, what it wants. Who cares about God and His worship. Hence I will let my young children read "church books" during Mass, but never have snacks. That one stuck with me.
Matthew