Age Appropriate 'SEX' Talk

So DD1 (6) came home last week saying I know how to spell sex and proceeded to spell it out and got all giggly & shy when I asked her how she knew that word and if she knew what it meant.

I played it down and just said it's an adult word and she told me an older girl had told her it meant when mums & dads got married.

This morning she got all funny again when she said she had written sex in invisible ink then started crying and saying "I need to tell you something".

I asked DD2 to leave and she said that her friend at school said that mums & dads got married, did sex then get a baby then they get married over and over again to have babies.

She seems quite scared to talk to me about it thinking she's getting in trouble because I said it's an adult word, where do I go from here.
I don't want her to worry about talking to me about these things and at the same time I don't want to make a huge deal out of it.

I don't want the other little girl to get in trouble and I'm not really sure to what extent she knows any more than marriage, sex, baby.

I thought this conversation would be well & truly years away.... HELP!!!!

I was about her age or younger when I first remember having that kind of talk with my Mum. Just small stuff because I'd hear things from bigger kids, I'd ask what things meant and Mum or Dad would answer in simple terms with no fuss.

I remember watching Hocus Pocus and they were laughing at the older brother for still being a virgin. I asked what a virgin was and Mum just said "someone who hasn't had sex." Another time I heard about condoms (no idea where) so I asked Dad what they are and he just said "something a man wears when he doesn't want a baby." They were simple answers that I was happy with.

Wroth regards to actual sex, I'd steer away from it being an adult word - I think it's ok for young kids to say the word, you just need to tell them it's not necessarily appropriate to talk about with their friends at school. I'd tell her that even though the word isn't "adult" and she can talk to you about it whenever she needs/likes, it means something that adults do and therefore shouldn't be talked about with other kids. At her age, telling her it's what adults do to have a baby might be enough for her. You can decide later how you want to approach it - something adults do when they love each other/because it feels good/etc.

I would explain it's a word to describe what adults do to have a baby. It isn't a naughty word, just one to describe a grownup thing, like having a job. So not something she needs to use at school at the moment.

I also tend to answer questions rather than give unneeded detail.

We have had a reasonably detailed sex talk with DS who was probably 7 at the time, because he knew that people did something to make babies, he knew it involved a man and a woman, but he wanted to know more about the mechanics of it (so to speak!). To be honest, once I had explained it he said "OK, that's what <insert friend's name> said as well". So I think by avoiding it you let them be informed by other people. I was impersonal with DS and tried to explain it from a simple biological viewpoint. E.g. the man has a seed, the woman has an egg. The penis goes into the vagina, which is how the seed reaches and fertilises the egg, and that grows to become a baby. I also explained that the process is hard to understand because it only works when the body is turning into an adult because the body works differently then. He seemed satisfied and hasn't really asked anything more.

Recently, DS was using the word gender to describe whether an animal is a boy or girl, but in animals the word sex is used and gender is for people (because it has physical, cultural and psychological elements), so it really is just a word. I use it all the time with DS (because I work with animals and he is interested in my work) and he understands it has two different meanings. I don't personally think 6 is too young to understand the basics. The whole emotional / pleasurable side of sex is something entirely different and appropriate for an older age (in my opinion)