We’ve asked the schools we’ve visited if they ever have issues, and most heads have acted as if they don’t know why we’re asking because obviously it’s not a problem in this day and age, kids don’t see difference as bad and it never comes up etc.
I'm aware people mean well, but this is such a tedious response.
FWIW, my DD has two mums. She attends a small rural primary school with very little diversity of any kind. When she started some of the children would run up to her in the playground or us at the gates shouting 'two mums! two mums!' and for a while we were a nine-days-wonder with the kids as the little ones would ask DD if she had two mums and the older ones would explain that you can have two mums.
None of this was malicious; it just is what it is. Small children will and do remark on anything they happen to notice and be interested in. It is far, far better to acknowledge that this happens than to pretend it doesn't - and IME, schools can be bad at this. Of course, it wasn't awful for DD to hear people shouting 'two mums!' (because, of course, she does have two mums!). But it could have upset her if it had struck her as a taunt, and so it did need managing, and I did need to explain to a couple of children yes, she has two mums, because DD aged 4 wasn't really up to that.
Since then she has had one older girl who bullied her about being boyish-looking; we're not totally clear how much this was also about her having two mums (because DD wasn't quite clear but said it was). School took this seriously and moved DD off this girl's lunch table (they sit in mixed-age groups), though there's still a slightly 'off' dynamic there.
The other thing we've had is that some parents - and I don't think they even realise they are doing this - are conditioned (probably because we all grew up with Section 28) to think that 'lesbian' is a rude word/something 'we don't say in front of children'.
That's the sum of it for us, and I am sure it would have been easier in a more diverse environment. It's not bad at all, but it is also very silly to think children never need to navigate these issues, because of course they do.