Was I BU to get frustrated with kids screaming?

You only asked for the window to be closed it's not as though you demanded the children be executed.
i don't think you're being unreasonable for finding it annoying- you can't help what you find annoying and random high pitched screaming is pretty objectively annoying.
the hygienist was rude because she was at work and she acted unprofessional.

Hygienist was rude, yes, but children need to let off steam at their break time, it's a different kind of scream and the adults there will be able to know the difference (and on duty so able to see the children in question).
If you are neurodivergent and subject to this kind of sensory overload, you need to make provision for yourself with that. E.g. earplugs
(awaiting assessment, also a parent, have been a teacher and TA)

This, wholeheartedly this.
Primary age girls often scream a lot more than is necessary.

I've got 4 kids and find screaming really bloody annoying

The hygienist was out of line, not you. You asked to have the window shut. You didn't scream stfu at the kids.

A lot of people have dental anxiety and try to zone out during any work being done. When the stressor of really loudly screaming kids is added on, that makes it difficult to zone out.

Good grief, I'd have snatched that whistle and it would have been gone. Your friend is letting her kid behave like a brat.

Next time take noise-cancelling headphones. They are brilliant in all sorts of situations if you are noise-sensitive (I am). School playgrounds are always noisy though, and always have been - not much you can do about that.

Hygienist was defensive so it's definitely her problem, not yours OP. I'd assume that someone else complained about her daughter's screaming previously and now she's overreacting.

Fuck that. I'm not ND, anxious and couldn't adore my (now adult) DC more, but I'd have asked her if she thought it was appropriate outside her job. What on earth was her DD even doing there? The dentist should have asked her to tell her to stop screaming or leave too.

I think the hygienist overreacted, not you. Lots of people don't mind children playing, even noisily, but draw the line at screaming/ squealing. My DM can't stand it and always used to tell us off. I'm less sensitive but do tell my kids it's unnecessary. The hygienist was obviously just touchy about her parenting choices. You weren't to know it was maybe her daughter!

Yanbu i struggle with loud noises especially when my anxiety is high, I don't see why they couldn't just close the window for a few minutes why you're there they can open it again when your gone and yes the hygienist was rude and unkind.

You were absolutely not unreasonable in the slightest. That would have set my nerves up on end. You were well within your rights to request that the window be closed and you are well within your rights to submit a complaint.

The dentist is 100% spot on. What if one day there is an actual issue and their screams are ignored?

I think the hygienist was very rude. I wouldn't let my children yell like that if they were playing outside at home. Maybe some laughing and chasing but nothing like that. I particularly dislike noisy children - and I particularly dislike parents who drag their children round supermarket where the child is making some non-stop high pitched keening noise which the parents seem to treat as totally normal. I can't imagine what they are like at home.

I think you were reasonable. You were in a tense situation and the noise made things worse. The hygienist should be used to people being on edge. She was rude.

This. Who the hell wants to listen to kids screaming and screeching. That's not playing, isn't surprising she thought it was her own kid!

I can't stand the sounds of kids screaming either. But if I express annoyance or irritation about it,you'd think that I'd said that I'm a child murderer.

Hygienist was rude because it was her own daughter doing the screaming

I was going to say something similar
... but I've only ever met two dental hygienists. However they were both snappy and frightening. Now I see I'm not alone.

This quote did not come through to my post for some reason, but was the one to which I was replying.

There is a huge difference between children being noisy while they are playing in the playground and screaming and you know this very well. Most people like the first sound but thè second one can really upset people. First, people like me, who have problems with their ears, it can actually physically hurt our ears. Most people will say that the sound goes through their bodies. A teacher or dinner lady should have stopped them. As for @Salumthecat, she was nervous enough already, as are many people when in the dentist's chair, and that noise was making her feel far worse. Nobody in their right mind would want to stop children playing but there still need to be boundaries in place.

I think the hygienist was so rude and OTT. Why she so defensive? You didn’t do anything wrong. I think when you’re a sensitive person (like you and me!) that struggles with things like too anxious, too hot, too cold, too loud, too bright, you end up having to ask for accommodations where other people would just shut up and put up. And as a result of having to ask for things like ‘please could you close that window/blind’ you get some people’s backs up, and get into these odd social encounters, that then leave your head spinning. Which is such a shame cos the same people who don’t care about the noise etc are the same one who wouldn’t give this encounter a second thought. Whereas the sensitive people are the ones who get into these conversations and then are left to overthink them too 😩
And also I have kids, and adore kids, but can’t bear the screaming as it just sets me on edge

Wtaf I have kids and that would do my head in! But it’s not up to the dental practice to tell the school to make the kids be quiet at break time? They should have just closed the window without snide comments. How on earth would the hygienist know if her own daughter was screaming? Bizarre.

I absolutely can not stand screaming and I think you did nothing wrong.

Honestly op why do you even care what they think anyway? Screaming kids are awful I agree

It sounds like it was the dental nurse not a hygienist. But regardless they sound like a gobby annoying person. You didn't say anything wrong, don't overthink it and also no need to apologise to someone who behaves like that. I bet the dentist is used to that nurse speaking that way and has had enough of them too!

I would make a complaint against her