OK. Here we go… You want to write a contemporary children's best seller, make it as dysfunctional and as negative as you can, avoid adventure (unless it is escaping from abusive parents/carers), make sure it is set in an orphanage, ensure there are plenty of zombies and threats of death, ensure that there is no hope (but indicate that hope might be forthcoming in the sequel) and ensure that there are enough adult themes that children want to pretend that they understand but don't to keep them mystified.
What is wrong with good old fashioned adventure and fantasy?
I'll tell you what.
Modern children's writers do not write for children, they write for publishers who demand formulaic best seller books which only satisfy the accountants.
Modern kids don't want to know about Rob Roy or Ivanhoe or Treasure Island or Swallows and Amazons even because they see them as dated and boring yet the irony is that all of the above mentioned books involved action beyond sitting in front of a computer screen bitching about having nothing to do except bitch about the world they can't really be bothered to take part in.
Schools are equally to blame… They judge children on their ability to read words rather than their ability to understand what they are reading. Schools these days are about awarding points for being seen to do things rather than the actual ability to do them.
I'm re-reading Stig of the Dump, its great.