I agree with paperbackwriter, there does seem to be a possiblilty that the primary curriculum could be trimmed a little. I don't think that it is impossible to make time to catch up in essential subjects, it might mean forgoing some things that they enjoy such as studying Greeks and Romans, fossils, dinosaurs etc. etc. My children all spent many hours on these and I have wondered if it was always time well spent.
Perhaps they could forgo the mask creation, model making etc and be given really interesting and informative books about these topics? Then those who are really interested can take it further (this might encourage reading in boys too).
Perhaps the long-term damage will be in the lack of exercise and poor nutrition that some children will have suffered throughout the past year.
Secondary school is a different matter, though I agree that some of the more advanced GCSE maths is absolutely irrelevant to many children. The same may be true for other subjects. It is a shame that children don't read more, so many less important topics could be covered by reading well written interesting books if only they could be successfully encouraged to engage with them individually.
I don't think the secondary curriculum is any different. Some subjects, such as languages and maths, are linear, but most aren't. Don't forget that many pupils in the UK don't learn any geography or history (or other subjects) after the age of 14 anyway. There are arguments about providing a broader curriculum (I'm not gong there), but I seriously don't think adults suffer that much from not learning about the Tudors or town planning in Nigeria (or whatever). Ask yourself how much content of the secondary school curriculum has ever been of any use.
The main purpose of learning in Key Stage 4 is to pass exams, which open the next door and so it goes on. There is absolutely no reason why pupils of a given age should know a certain body of knowledge, other than to pass exams. There is already support for abolishing GCSEs since the school leaving age has effectively been raised to 18.