TIL that Deary is a very popular author. He has a series called Horrible Histories. I never heard about this series before.
When asked about the radical undertones of his work, he told the newspaper: “That’s the way snaky propagandists do it. I’m poisoning the minds of children…yes!” (telegraph.co.uk)
Terry Deary has said of historians: "They are nearly as seedy and devious as politicians..They pick on a particular angle and select the facts to prove their case and make a name for themselves... They don’t write objective history... Eventually you can see through them all. They all come with a twist." (wiki)
The assumption, of course, is that objective history is actually possible. There have been fundamental changes in how we few history in post-Rankean (i.e. postmodern) historiography. I'm not going to take a side on the issue (objective v. subjective history), but it is important to keep in mind that the possibility of 'objective' history is highly debatable.
That said, there is a big difference between subjective history (the postmodern understanding of it) and what I'd call 'bookstore history.' There's nothing wrong with bookstore history – books designed to be sold to a popular audience – however most come with a 'point' or a convenient conclusion/theme that makes the work easier to comprehend. Subjective history, on the other hand, is simply history that is inexorably influenced by the perspective and world-view of the historian him/herself. There isn't always a point, and there's rarely a convenient theme or conclusion that wraps everything up nicely – most books like this leave me with more questions than answers (and that's a good thing). Some historians do toe the line between these categories (Mark Kishlansky or Bernard Porter come to mind), so the demarcation is by no means clear-cut.
To conclude, Deary is actually committing the error that he claims all historians are committing, if he isn't just being completely sarcastic and satiric – something he is known to do. Unavoidable subjectivism in history is not the same thing as propaganda, politicization, or 'making a case' for a subject.