Daughter (10) says she wants to be a boy

What does being a boy mean to her?
What does being a girl mean to her?

Is she getting unsettling male attention now that she is in puberty? It's understandable that some girls might feel safer as 'boys' as an attempt to shield themselves. Being female is scary in that respect.

OP you sound like a brilliant Mum and clearly have a strong relationship with her.
A few thoughts...
If some/all of this may have been triggered by starting her period is there anything you can do together to make them better? Is she struggling/embarrassed by having to use San pro would period pants be any better? Similarly if she’s unhappy or physically uncomfortable with breast development/wearing a bra would a crop top or sports bra help?
I think I’d find it really hard to call my child by a different name because that would need a conscious overriding of what I naturally call them and the way I think of them but I wonder as a compromise of a gender neutral nickname would work?
Totally agree she needs to see lots of different types of women to know we're not all pink and frilly. There are lots of sports women who are amazing- Nicola Adams springs to mind but rock climbers, cyclists, snowboarders are often not very girly and there’s some brilliant films about them. Have a look at the Banff Film Festival website for examples.
I also bought a book for a friend’s daughter called “Folk Tales for Bold Girls” which has folk stories from around the world where girls are strong brave and independent.

Wearing dresses is not anything to do with being a girl

I would want to know what's going on with her friends. Whether there's anyone else exploring their gender in her circle or her sphere of contacts. Social contagion is fierce amongst young girls. I'd monitor messaging apps as well as internet usage. And no phones in the bedroom at night.

My son is 10 and has wanted to be a girl since as soon as he could squeeze himself into Elsa and Ana dresses.
I don’t think he could explain “why” though...it would be possibly quite overwhelming for him to have to do so...
Sometimes he wants to talk about it and sometimes he doesn’t but it’s all ok - sometimes he says he is gay and sometimes he says he knows he is physically a boy but in his head he is a girl...just be open and loving and say to her that you are here for her and any time she wants to talk about it she can...and reassure her that you will always love her no matter what she decides to do ..and even if she doesn’t want to decide either way that’s ok too x

My daughter was the same at that age. She dressed as a boy, had her hair as a boy and I of her happiest moments at school was when a member of staff told her off for using the girls' toilets, because she was mistaken for being a boy.
I drew the line at calling her Josh because I wanted her to be loved for who she (or he, if that's how it panned out) was.
She grew out of it all over the following few years.
Ironically her older brother is trans...
We are what we are. My advice is to just try and roll with it all.

I wanted to be a boy when I was coming to terms with my sexuality. Just keep asking open ended questions about why she wants it. It’s very possible homophobia or biphobia at school or could be something simple as her not wanting periods or finding them painful.

I have read this mostly the other way but I think there are a lot of online suggestions to children to get one parent to tell the other one "because they are afraid how they will react". Usually they ask mum to tell dad. Do you think she's being "talked to" online?

I wanted to be a boy, lots of us did. I think it was easier in the 1970s as wearing your brother's hand me downs was fairly normal. Don't freak out, it's likely work out ok.

I'm wary of going down that route straight away. While that is entirely my opinion (and biological fact) I don't want her to be in the mindset that she's stuck and feel desperate at any point.

She's already in a stuck mindset thinking that she has to be a boy to be able to feel comfortable in her skin, poor child. She's a girl, a female child who has the option to be as gender neutral as she she likes. Beyond that, what does she think boys do or are that is different from girls?

Show her that it doesn't matter whether she is a boy or girl but that what matters is her interests, doing well at school, hobbies, friends, her health, if she is kind, if she can make a difference in the world by helping others... boy or girl, who cares? There are more important things in life.

She might like the She-Ra cartoon reboot. It's very good and has a nice, unforced diversity in the characters around sexuality, colour and body shapes.
Re periods, my DD started at a similar age. She hates having periods but we've found period pants to be the least fussy or intrusive way to deal with them, also pretty environmentally friendly.

Thanks, Charlie, that's good to know.

I'm sad to hear people dislike periods (not judging, it just seems sad).

If there are side effects, period pain, etc, that should of course be addressed, but is it seen as bad in itself to have periods? I love mine. I appreciate this may not be universal, but I love the rhythm of it (and no longer have pain, thankfully). A healthy, natural part of being a woman (girl, in this case!).

How long exactly has she felt like this ? Could she just mean she wishes she was a boy so that she doesn’t have periods ? I would take this very very slowly and wouldn’t overload her with information unless she asks.

"What makes you think that?" is a favourite question in our family. The kids can't always tell us why, but they can often explain what might have influenced their thinking. That's what I would ask my kids in this situation. We're very STEM-focused, so we would also talk about sex being immutable, but that that doesn't mean we need to fit neatly into the boxes society makes for each sex.

But you sound like you've got a loving relationship with your DD and a sensible way of approaching the issue, so I'm sure you've got this.

Like pp suggested, I would also look for strong role models, both in lesbian and bisexual women and ground-breaking ones. My mum got me a dictionary with the biographies of 100 women who made important contributions to the world at this age, and I really loved reading about all these women doing things that women of their time just didn't normally do. It helped me to know that I wasn't limited by being a girl (outside of some narrow biological contexts).