One minute they're four, and wondering where babies come from and how they get there. The next, they're 14 and going to junior discos where "getting off" - i.e., deep French kissing - with complete strangers is popular.

And you realise that, since answering their four-year-old questions, you've hardly discussed sex with your children at all.

Every parent knows the standard advice: answer children's questions about life and love honestly and age-appropriately. But if they don't ask, the years can easily slip by until they've reached puberty - which nowadays can come at 10 and 11.

Suddenly you wake up and ask them some questions to find out what they want or need to know. Then you discover you've nearly left it too late, as with much eye-rolling they let you know that your queries are just plain embarrassing.

So what are the basics that parents should tell their pre-teen children about sex and relationships? Most primary schools are on the verge of introducing full-fledged RSE (relationships and sexuality education), but this doesn't let parents off the hook. Indeed, the Department of Education's RSE unit is at pains to point out that parents are seen as the primary educators of their children, and view the RSE programme as supporting parents, not the other way around.

It's evident that parents' approaches to this subject vary widely, as anyone who discovers that their child has just revealed a few facts of life to another will know. It's equally obvious that - no matter how much you try to control the books they read, the TV and movies they watch - they are going to hear lots of information sooner rather than later, probably in the playground.

As it turns out, the experts seem to agree that you don't need to give children too much detailed information up to the age of 11. However, by then they should certainly understand about sexual intercourse, conception and birth, and about the changes that will happen to them in puberty, e.g. menstruation and wet dreams.

It is all much easier if you start at the very beginning, when they are babies, by naming all the parts of the body properly, so that you don't send out the signal that there is something shameful, or odd, or funny about certain parts of the body. We don't usually have slang or baby names for arms, feet and legs, after all. Fionnuala Kilfeather of the National Parents Council (Primary) says it's useful for parents to get comfortable with using words like penis and vagina when their children are tiny infants, long before their children are asking questions.

Telling children the simple, basic facts when they're under six or seven is "so much easier than taking a deep breath when they're 12", she says. "In reality, telling the facts is really a very small thing," she adds, stressing the importance of a dialogue from early on about feelings and relationships.

Emer Egan, a senior inspector in the RSE unit in the Department of Education, says the other advice is to answer children's question simply but honestly when they ask them: the thing is to avoid jumping ahead three questions down the line and offering more information than the child is likely to take in.

She points out, for example, that the word "orgasm" is not used at all in the RSE program for primary schools, despite a recent controversy in which the Department was accused of using pornography in schools. It is true that a number of books suggested as resource material for children, parents and teachers who are involved in RSE do explain just why mothers and fathers make love to make babies - that is, because it's fun. (The average nine-year-old is inclined to regard sexual intercourse as "gross" and is a little mystified when told that adults do this because they like it.)

Psychosexual therapist Mary O'Conor says that while there's nothing wrong with telling children this (most of us nowadays are trying to promote a positive view of sexuality), it may well be more detail than most children under nine want to know.

The RSE programme is much much more comprehensive than the old-fashioned, biological idea of sex education: it teaches children about sexuality in the context of individual growth and developing friendships and relationships. It points out that children are getting messages, verbal and non-verbal, from infancy about how people treat each other, and whether they're comfortable with their bodies.

The problem for many parents is that although they would totally agree with one of RSE's aims, "to enable the child to be comfortable with the sexuality of oneself and others while growing and developing", they cannot overcome their own unease with discussing sex because of their own upbringings.

Both Emer Egan and Mary O'Conor recommend getting hold of one of the many books that help you teach children about sex - the RSE resource catalogue has a list suitable for all age groups - and using it as the basis for discussion. They also recommend that you read through the book first yourself to make sure you're happy with the content, and happy that it's suitable for the age and stage your child is at.

A trip to your local bookshop will prove an eye-opener - there is a very wide range of books available.

Try not to react in embarrassment when something sexual comes up on TV, says O'Conor. If you flick channels, or leave the room, or switch off abruptly, you're simply sending the message that you're uncomfortable about sex. However, she adds reassuringly, it is quite normal for parents to feel uncomfortable about discussing sex, especially in the teenage years.

With all this in mind, the Department of Education and the National Parents Council are currently piloting an RSE programme for parents in seven areas of the country, with the hope that it could eventually be offered to parents all over the State. The aim of this, Egan says, is to help parents carry out their job as primary educators of their children in the relationships/sexuality area. Many parents have shown strong interest in it, which isn't surprising: surveys have shown that while most parents believe they should be the primary educators on this front, most children report that they do not get their information from parents.

Not all primary schools in the State have yet introduced the RSE programme, but you will know about it when your school does: an essential part of the programme is involving parents on drawing up RSE policy for individual schools. If you want information about it, ask your school, or contact the RSE service at the Education Centre, Drumcondra, Dublin 9.