teacher I actually disagree with the idea that children should be pushed to more complex texts in order to enable them to meet the requirements of the KS levels - sorry if that wasn't clear.
I was put off reading Dickens for over 20 years because I started on it far too young - I was capable of reading it at 10, but not of really enjoying it: and I think this is a huge danger if we push children towards reading classics etc just because they can. It's much better that they are engaged and enthusiastic in their reading - plus, as you say, very little of fiction written for adults today is actually as grammatically/structurally advanced as is expected of primary school children.
Unlike Aeroflot, I didn't personally find the mini-mocks difficult, as a 41 year old woman - but I suspect, despite being a passionate reader and writer at 10, I would have found them trickier then: a lot of the types of questions they were asking were things we did in preparation for moving school at 13. I realise, though, from talking to contemporaries, that my first school was relatively old-fashioned when it came to grammar - from memory, we had at least an hour a week that was just punctuation and grammar, and the parsing of sentences, identifying all the parts of speech and sub-clauses blah blah blah. We also continued to study grammar at secondary school, up to GCSE, but this was just reaffirming the knowledge we had already - in those early days, though, there was a grammar chunk on the English Language paper: not sure if that's still the case though.