What are talking about, Anonymous?
The story - Jews being hidden from the Nazis by very brave sympathisers - is not in the least implausible. Nor does it contradict any of 'known history'. So you begin with two lies.
Plus you announce "it has already been established' - beware of the passive. Anyone making a statement like this is probably trying to put one over on you, and so is Anonymous. There was never any secret that Anne Frank's father did remove a few details which, in the mid twentieth century were thought unsuitable for the general reader - they included Anne's thoughts about sex. Anonymous will no doubt be pleased to know that the entire, unedited Diary is now available, and that the diary was authenticated
"The Netherlands Institute for War Documentation commissioned a forensic study of the manuscripts after the death of Otto Frank in 1980. The material composition of the original notebooks and ink, and the handwriting found within them and the loose version were extensively examined. In 1986, the results were published: The handwriting attributed to Anne Frank was positively matched with contemporary samples of Anne Frank's handwriting, and the paper, ink, and glue found in the diaries and loose papers were consistent with materials available in Amsterdam during the period in which the diary was written."
https://en.wikipedia.org/wiki/The_Diary_of_a_Young_Girl#Authenticity
Anonymous, you say: "Often extremist groups will fictionalize a story and create a symbol of that story. Typically someone who has strong credibility and draws sympathy...like a child."
Really? how about a couple of examples?
And what do you mean by 'extremist groups'. The diary was authenticated by The Netherlands Institute for War Documentation, has been published by mainstream publishers and is available for study in schools. None of this is characteristic of anything produced by 'extremist groups.'
On the contrary it is the extremist groups who attempt to challenge the authenticity of the diary:
In 1975 David Irving who "the High Court in London in 2000 declared to be a Holocaust denier, racist and antisemite"was sued for claiming the diary was a forgery and his publishers had to remove this statement from his book.
In 1978"Ditlieb Felderer, an Austrian Holocaust denier, wrote a viciously antisemitic pamphlet called Anne Frank’s Diary: A Hoax in which he attacked Anne’s character. In chapters called “Teenage Sex” and “Sexual Extravaganza,” Felderer called Anne’s relationship with Peter the “first child porno." (worrying for so many reasons).
In 1996 David Irving tried again - although admitting he hadn't actually read the whole diary.
So the extremists and the Holocaust deniers are the ones casting doubt on the diary's authenticity. Do you agree with them?