I watched this and work in a related field. My thoughts are this - the main perpetrator already had a diagnosis of schizophrenia by the age of 12 and struggled to deal with emotions as discussed by her mother. The other girl was easily led, as confirmed by her father.
When dealing with young offenders in England you firstly have to take into account whether or not the crime is considered sufficiently grave. Generally offences which are deemed to be very serious - and this is. Secondly the likely sentence to be received by the girls. Anything over two years detention leads to a case being tried in the U.K. Crown court. As these girls were charged with attempted murder any sentence is to be in excess of that. I'll admit I'm not aware of the process of the courts in the state they are currently being tried, but it doesn't surprise me that they are being tried as adults.
Secondly, there was premeditated actions by the two girls and this wasn't their first attack on this victim. There is a history to this which is deeper than the documentary, read about it, as it is well established as part of the prosecution case.
Also, the slender man documentary was very one sided. The views of the defendants parents and their backgrounds are relevant, of course, but it adds an element of sympathy which wasn't able to be counterbalanced by the views of the victim. Why was this? It's actually because the victim refused to be part of the documentary. In fact, both her and her family wanted nothing to do with the media due to the trauma the poor girl has suffered (and is still suffering). This lead to the documentary presenting a one side view. A documentary with only one view is so overwhelmingly biased that it's almost irrelevant.
Those girls were young but were deemed old enough to understand their actions. It must be a difficult case for their defence lawyers as this type of crime is so rare in people this young, hence them scrambling for mental health issues as to provide some reasoning for the actions of these girls.
But their actions were abhorrent and should result in very serious punishment as it was down to luck that the victim didn't lose their life.