In theory, education in Britain in the 1930s wasn't quite universal and wasn't quite as thorough as it is today - but school was free and mostly compulsory from 5-14 and there was a strong bias towards literacy and arithmetic (religion, music, art, etc. were still taught - but for most making sure all children were literate and numerate was the priority) making for an expectation that the 'average' child could read and divide at a reasonably high level.

HOWEVER - that middle class goal for the working classes wasn't always, or even regularly, met. Parents worked long hours and while fertility rates declined dramatically around this time many children still had to work at home or in industry - they weren't being sent down the mines anymore, but exceptions to education requirements applied to agricultural families and truancy officers and local authorities were stretched thin enough that enforcement was very patchy.

When a child was getting the expected education then yes, short and child-friendly novels such as The Hobbit would be a welcome break from school books (most learning was done from incredibly dry classics or overly simplified teaching books that lacked content to incentivise reading) and offered a genteel alternative to most youth literature of the time (which was pitched older and... pulpier.) For context, The Hobbit is currently taught at KS2 (ages 8-10) in British schools, but many are introduced to it younger as it remains a popular children's book - rated as Britain's 25th most popular book (Lord of the Rings is number one) by the BBC.

But all this about the nonexistent 'average' child doesn't explain what young Rayner Unwin said about 5-9 year olds. Rayner was most decidedly NOT anything like our imaginary average child. He was the son of one of Britain's leading literary agents (Sir Stanley Unwin), and by this point was already working for his father (his review of The Hobbit was one of many), and the 5-9 year olds he knew were also not the 'average' working class boys and girls I spoke of earlier. It's also worth noting that he was specifically defending the lack of illustrations in a children's manuscript, saying the maps make up for it.

As a post script, I'll just say that Rayner Unwin would go on to publish The Lord of the Rings and was the first publisher to accept Roald Dahl's children's books - at the advice of his own daughter. So while young Rayner's view of the average child was skewed, he wasn't that far off - and as a ten year old in a family that put their children to work as literary agents from the age of six we can cut him some slack. And he proved that he did know what kids like - he published some of the most popular children's books in history!

To sum up - his view was optimistic of British children at the time and today, but not wildly so considering his personal context.