Greetings!
I work on the issue of child soldiering for a living, hopefully I can clarify some of your questions.
TLDR: Events in the film are definitely sensationalized, but some do reflect the actual experiences of some child soldiers.
Beasts of No Nation is a feature film, not a documentary. It is based on a 2005 novel of the same name by Uzodinma Iweala, a US-born author whose mother Ngozi Okonjo-Iweala is the former Finance Minister of Nigeria. Iweala explains in a quasi-interview conducted by his publisher (which was included in the 2006 Harper Perennial edition of his novel in the "P.S." section and likely future editions) that he was inspired to write his novel after meeting the former-Ugandan-child-soldier-cum-activist China Keitetsi and reading an article on child soldiering in Newsweek magazine. The American director Cary Fukunaga adapted Iweala's novel into a feature film, adding and deleting certain scenes/elements, which again come from the imagination of Iweala.
Is the film/story historically accurate?
No, insofar as the film is set during a postcolonial civil war in Ghana. No such event has ever taken place in Ghana, nor have child soldiers ever been known to operate in the country post-independence. That said...
How sensationalized or "white washed" is it?
Some events depicted in the story are more common than others. Most young people who are classified as child soldiers by the UN and the umbrella NGO Child Soldiers International have no direct involvement in armed conflict, as the character Agu does in the film. Many perform secondary roles as porters, cooks, camp guards, etc. Agu's experiences in that regard are not necessarily typical of child soldiers.
However, in nearby Liberia and Sierra Leone children were used as front-line fighters by both government and non-state armed groups. Moreover, one technique used by armed groups to desensitize child soldiers to violence is to drug them, which was vividly depicted in the film. And finally, many child soldiers (boys and girls) are vulnerable to rape by their commanders and other soldiers.
If you have any other/more specific questions, I'd be happy to try and answer them!