To be upset that toilets at primary school turned mixed sex without being told

I believe they legally have to provide single sex from y3 upwards.

The knowledgeable people on the Feminism: Sex and Gender board will be able to tell you chapter and verse on this and signpost you to guidance, template letters to send etc.

There is very clear laws on toilets so depending on their age you maybe able to use the guidance linked below.

You also need to be clear how the toilet(s) is set up a fully enclosed toilet or cubicles. Mixed sex must be fully enclosed with hand washing facilities not cubicles.

By law there can be no children with a gender recognition certificate so no child can legally change sex so effectively that should not be an issue.

There's information here womansplaceuk.org/2018/11/12/mixed-sex-toilets-ins-chools/

Ks1 - fine.

Ks2 - should be single sex. Girls in Y4, 5, 6 starting puberty need girl-only spaces.

At my school, the girls and then all children (boys & girls) in nursery, reception and year 1&2 shared toilets as they were closest to the year 1&2 classrooms.
Then boys had their own separate toilets from year 3-6.
I think the issue is if there are older boys in year 6 sharing toilets with reception children in my experience. It's extremely odd that this wasn't brought up with parents beforehand though???

most haven't hit puberty until secondary,

I don't know about this. When I was at junior school we all got changed in the same classroom, but there were girls and boys toilets. There was an unwritten policy that as girls started to feel uncomfortable taking our tops off in front of the boys, we could slip off and get changed in the girls' toilet. I remember vividly not really understanding why some girls were dong this until midway through year 6 when I became grateful to have the option. Also a few girls had already started their periods by year 6 .

Why can't it be single sex/gender? I'm missing something?

Considering primary-aged children change for PE in the same room as the opposite sex in KS1 and most haven't hit puberty until secondary, I can't get too worked up about it, especially for the little ones. They're not at risk of sexual assault like they would be in a secondary school or in a bathroom shared with adults. As long as cubicles are private, they're reasonable.

I wouldn’t be happy at all. I get that they are there to provide somewhere that doesn’t cause discrimination and for all children to have a toilet that they feel caters to them. It must be horrible to not know what toilet to use if you are transgender, people will complain should they not agree with them being in the f toilet for example. It’s not a disability so no good telling them to use those instead that’s insulting and alienating. I do however feel that the same standard should be held for those who identify as m/f and they have somewhere they are comfortable to go. IMO there should be at least one choice of m/f for that reason. It took us a long time to get my DD to use the toilet at school if she needed a no 2 I know she would not go at all if it was a mixed sex toilet. The school should have informed parents. Probably a council wide thing in all schools in the area I’d think.

KS1 toilets at dc’s school have always been mixed sex. Not sure about all of them for KS2 but I know there is at least a ‘girls toilet’ as obviously at that age some are starting to go through puberty, get periods. I couldn’t be worked up about this as long as individual cubicles are private. My work ones have long been mixed sex and honestly I’ve never really thought about it. They feel nice and safe and I’m no more concerned with my male colleagues hearing me have a shit than my female ones. 😂

Have you asked their reasons?

Toilets at one DC school have always been mixed sex, every pair of classrooms shares a cloakroom in-between them and in each cloakroom are two toilet cubicles.

Other DC school used to have girls toilets and boys toilets, girls were at one end of school and boys were at the other end (it's an old building and 100yrs ago they had the girls side of school and the boys side of school). During covid when they had to minimise DC moving around and maintain bubbles they had to make them single sex and found it worked much better as DC no long had to traipse to the opposite end of school to go to the boys/girls toilet and could instead just use the one nearest them.