Yes but when that's where they've been it's a big deal to change their world completely! Can't you understand that?
Very, very gently - I think you need to try to reframe how you think about this. It's not their complete world. It's their school, and lots of kids change schools.
You are their world. You don't have experience of the state sector as a parent, and I think it's become a bit of a grim
bogeyman in your head. It's certainly not 'another world' - and there will be pluses and minuses to both. In a larger and more diverse environment, there are more ways for your kids to find their own tribe, for a start - my own experience of private education (primary and secondary) was that smaller class sizes and less diversity encouraged conformity, and the punishments for those who didn't conform were harsh.
I loved my school, and my school friends, and always assumed I'd send my kids private but having just chosen a secondary for my own eldest I massively surprised myself.
The facilities at the private schools were (mostly) better (one or two surprisingly not quite all that) and by and large the kids had slightly more polish. And we looked round a few state secondaries that I hated, but two I really liked and that DS1 loved. He's so excited by the opportunities it offers, some of which aren't on offer at the £25k a year private options we looked at, and the huge enthusiasm of the staff members he spoke to, and the school's general ethos to education.
I get that you're devastated right now. You've obviously had a huge and very negative upheaval, and I suspect some of this is playing out here. Changing schools is going to be tough for your kids, and if it's doable it may be worth looking at options slightly further afield for a cleaner break?
But look at the schools with an open mind. Some of the buildings will be old and tatty, but that really doesn't matter. Listen to the staff and the kids. Trawl their website for the extracurricular stuff your kids will be enthused by, and use that as a selling point. Our comp has a brilliant performing arts programme and its students have performed at the National Theatre more than once, for example. It massively sparked DS1's imagination and he's so enthusiastic about what's on offer at the school (also caught his interest: rock school, Marvel film club, chess club, language trips to France and Canada, great basketball team.)
You've given your kids a good start that will set them up for whatever comes their way next. Doing their GCSEs and A levels in the state sector will, all things being equal and their commitment to education, stand them in good stead for university admissions. Good luck to you and them.