Explaining DN transitioning to my DCs -quick advice needed, please!

[IllMetByMoonlight]

My DN (17) is paying us a surprise visit on Monday, I've just found out. Her family live abroad and owing to a historic falling out, contact between me and DB has been scant and tentative for the last decade (although I hope with all my heart it's been resolved-ish), so I don't know DN at all well, and we haven't met for 7 years. We have been in sporadic contact on FB where DN has posted general information and memes about transgender issues. This evening I asked whether these relate to DN's own life or are in solidarity with other transitioning friends, and if the former, which name and pronouns I should be using. So glad I asked, DN super relieved and supplied new name and pronouns within seconds.
My question relates to my two DDs (6 and 3) who have idolised their cousin from afar for years, never having met DN, and who are so excited about DN coming to stay. But they know DN by birth name and have only ever seen pictures of DN in prom dress and distinctly "wrong" (for DN) gendered clothing; they think this fairytale princess is coming to stay. I've got a day to explain to them what is happening in DN's life, in a way that it will seem totally obvious and reasonable to them to change name and pronouns. Any tips as to how I can get close to this? I just want DN to feel received in congruence with his own lived experience of himself.
Apologies if I am expressing myself clumsily, I haven't had occasion to become acquainted with the "right" way of speaking about this, so I hope I have not inadvertently been insensitive.

[lifeisunjust]

I don't think you should explain anything to your children. Your niece is more likely to be a lesbian uncomfortable with her sexual orientation than think she is a man (which she can never ever be, your sex determined as one of 3 at birth) and surgery, make-up, clothes will never make her a man.

I think you should tell your children ZERO. If your niece turns out to be, as the majority do, to be lesbian, would you tell your children that?

If you have to tell them anything, don't tell them there is such a thing as female brain or male brain, it is lies Science has backed up as lies. If you have to say something, say your niece prefers male clothes and male names. That is ALL they need to know.

[dancemom]

Lifeisunjust - seriously??

[llhj]

Well transitioning is not an easy thing to explain to children that small. I would just say that this man (insert new name) is coming to visit l and leave it at that.

[OMGtwins]

They're 6 and 3 so don't actually need that much information because they don't have all the baggage around this that grown ups do. Just tell them that your DN has decided that she feels more comfortable being a boy, and that her new name is x, and leave it that. We have 3 year olds and a friend of ours has gone the other way. We just told them Uncle X is now Auntie Y because she feels better that way. They didn't ask any more questions. They might later, but it's not needed now.

HTH xx

[lifeisunjust]

Yes seriously. Saying the cousin like boys clothes and a boys name is sufficient. Young children do not need to know the ins and outs of sexual orientation or the mental health of others either or anything to do with gender dysphoria and usual accompanying personality disorder, for those who are actually finally diagnosed.

The other posters have it just right, no adult baggage for kids. Tell them straight the niece prefers boys clothes / names and that's it. As for transitioning, tell them zero, they don't need to know more.

[harderandharder2breathe]

Just say that DN would like to be called X and referred to as a boy during this visit. Kids so young are pretty accepting of whatever they meet with

[Finola1step]

As your dds are so very young, I would simply tell them that sometimes people like to change things about themselves. That it is wonderful that their lovely cousin is coming to stay and that their cousin chooses to be called X. Then leave it there unless the dc ask specific questions.

Just keep referring to how wonderful it is that their cousin is coming to visit.

[IllMetByMoonlight]

Thank you for your replies!
I think that between your good advice I'll be able to say what's relevant. Life, yes, she is gay, but that's not relevant to my question, I don't think. But the DDs wouldn't bat an eyelid between them as we have a number of gay friends, male and female. How do the DDs know? Because said friends are in relationships.
Thanks again :)