My attempt to rewrite the array at the level of the item I wanted to introduced a subtle flaw. I made it clear what data structure I wanted:

[
          'note',
          [
            'shopping',
            
              [
                'item',
                'bread'
              ],
              [
                'item',
                'butter'
              ],
              [
                'item',
                'beans'
              ]
            
          ]
        ];
[download]

but then the code above introduces an extra level of array referencing that is not acceptable:

[
          'note',
          [
            'shopping',
            [
              [
                'item',
                'bread'
              ],
              [
                'item',
                'butter'
              ],
              [
                'item',
                'beans'
              ]
            ]
          ]
        ];
[download]

it looks like I will have to write the child list at the level of the parent instead of rewriting at the level of the child.