If you've seen any documentaries or read any articles on how child soldiers are indoctrinated you've already seen this movie. Best part of it is Idris Elba as the Commandant who always turns in a great performance in whatever he's cast in.
So going by that logic, if I've read about any topic a movie chooses to tackle I've seen said movie? So I've seen Spotlight because I've read about the Catholic Church's deeply embedded acceptance of pedophilia? I've seen 12 Years A Slave because I've read about slavery? How reductive.
Also, not even a mention of the 14 year old boy that out-acted almost everyone in the movie; even, arguably, Idris Elba? I refuse to believe that you think that kid was just 'eh'.
And I suppose you thought the direction and cinematography was ho hum too. Seen it all before in those documentaries you've watched.
I'm honestly interested in why you thought this movie was redundant, as you seem to be implying.
Every year a movie like this gets churned out and a small number of people who don't understand acting cream over themselves about it. Same thing happened with City of God.
The kid did ok. Like everyone else in the movie, they were fit for purpose, but nothing outstanding.