I am hearing very similar about yr2.
My son is that age and is doing ok but he's very much the exception to the rule (being one of the eldest in his year and an only child i think are significant factors in this).
His class are wild and the other class in the same year are challenging but not quite as bad. Many are being treated as SEN but its more to do with behavioural issues steming from never learning the skill of sitting still and listening and then falling behind with reading / writing and that having knock on effects, rather than learning difficulties and proper diagnosis. There is an unusually high number in his year requiring extra support as a result.
And its not just his school. I have primary age teacher friends and friends with kids a similar age and they are all saying yr2 are the worst they have ever seen.
There is one girl in my son's year, who is the daughter of a former deputy head and very much minded towards education during lockdown. Even she is struggling and behind where she should be (she's a summer baby which has compounded it)
I don't know what the SATS are like for my son's school, but Ive heard they are down at other schools, and I do know that a number of parents in my son's year have expressed concerns about their child being below expectations. Its a good school and its not what you'd expect.
I know the class well as Ive helped out with them. Lovely bunch of kids, but yeah just wild. You could see it very clearly on sports day against the other classes. I reckon there's probably 6 of them who are about where they should be. The others are below what they are capable of, or struggling with behavior, for various reasons.
So my feeling is not to panic. It is an issue, but its across the board. Its hitting all levels of socio-economic background. Its undoubtedly worse in some areas, and even some parents who are trained professionals couldnt manage to teach their own kids adequately from home. Yes, she has a lot of catching up to do, but she's not alone and even if she wasn't directly affected the impact of others in the class who are struggling badly, has a knock on effect. If it really is across the year, she isn't going to be significantly disadvantaged. Schools will be forced to put more effort into that year and they will be watched carefully.