How can I add a sub-array structure in my results? $_POST
I managed to solve my own problem. It was a problem with the #tree.
I have to set #tree in the parents to true..

So if you want an array with the following structure:

array {
    ["parent"]=&gt;
     array(4) {
        [1]=&gt;
        array(9) {
          [1]=&gt;
          int(1)
          [2]=&gt;
          int(0)
          [3]=&gt;
          int(0)
          [4]=&gt;
          int(0)
          [5]=&gt;
          int(0)
          [6]=&gt;
          int(0)
          [7]=&gt;
          int(0)
          [8]=&gt;
          int(0)
          [9]=&gt;
          int(0)
        }
    }
}


I had to configure #tree for the parents (2 here), and not only for the children

['parent']['#tree'] = true
['parent'][1]['#tree'] = true


I figured it out after trying and reading https://www.drupal.org/node/48643 (Archived, but it worked for me ;) )