The Opium Smugglers (1948) is one of four books by Idriess marketed as the Australian Boys' Adventure Series. The others in this series were Headhunters of the Coral Sea (1940), The Great Trek (1940) and Nemarluk: King of the Wilds (1941). The blurb for Headhunters (on the DJ inside flap - but repeated in its advertisements in the other books) was "Here is an exciting book of adventure for all readers from nine to ninety." And these ages are repeated in other blurbs - e.g, the back inside flap of The Great Boomerang states that, for the three earlier books in this series, they are, "For Boys from Nine to Ninety." Idriess begins his Author's Note for The Opium Smugglers with these words: "This boys' book is for your dad as well as for you."
So the question is: Are these books suitable and appropriate for boys? And for boys as young as nine years old? First, it is recognized that these books were published in a different era - the 1940's - just as WWII was raging or had just concluded. Mores as regards the suitability of certain literature for children was a bit more nebulous than today. There was not the viewing codes for television - the content ratings of G and PG, M, etc - that we have now. In fact no television per se - which may have been a good thing for children then. But no doubt parental guidance for reading matter was just as expected as nowadays.
Yet there is also no doubt that some of the themes and graphic imagery in these books is adult in nature. Nemarluk has the killing of the Japanese from the Ouida; Headhunters has the massacre of most of the survivors of the Charles Eaton and headhunting sorties; and The Great Trek features an almost constant battle against "hostile blacks" with many of them being killed or wounded - all for the sake of ploughing through their country to reach a distant destination.
In The Opium Smugglers there is overt racism: racist terminology and racist stereotyping, e.g., Little Paddy is described as "a little black monkey" (p.5) and "like a monkey" (p.11), perhaps offset a little by admiration for the aboriginals' tracking and other skills. But racism is unfortunately endemic in Idriess - and many others of the era.
There are other things from a 21st century perspective that we would frown upon - especially for a children's book! There is smoking by many (e.g., p.11), but not limited to the adults (or near adults, if Jack and Dick, by their characterisation as "boys" themselves in the book - perhaps they were late teenagers or young men under 21?): Little Paddy himself is given cigarettes (p.155). There is the graphic mauling of Billy by a tiger shark (and, as an aside, there is the labelling of the poor old grey nurse shark as a man-eating threat to people by Idriess; a common but unwarranted misconception of the time) and his being doctored and sewn up by Cross-eyed Joe. There is the torture of Cross-eyed Joe and the two Malays by the Japanese crew. But perhaps most disturbing of all is the child abuse (torture!) of Little Paddy by Slinker, who half-throttles him, belts him with a belt on his bare bottom and ties him up. And then when this comes to light Little Paddy is laughed at by all.
Perhaps these stories could have been pitched at teenagers and above - who today cannot only read but perhaps unfortunately also see a lot more graphic material - but not nine year olds!
Finally, as to the truthfulness of The Opium Smugglers - I am inclined to agree with Dal McGuirk (Forum Q and A - Clive English, November 26, 2020) that this was a story based on Idriess's experiences but woven together into one plot.