Cannot add children in debugging helper

Hi!
 
I'm trying to write some debugging helpers for Qt Creator, but I've been struggling with children and subitems for the last few hours without success.
 
I reduced my issue to this simple case. Given a type
 

struct Foo
{
    int i;
    int j;
};

  
and the following debugging helper
 

def qdump__Foo(d, value):
     with Children(d):
         d.putSubItem('i', value['i'])
         d.putSubItem('j', value['j'])

 
Qt Creator displays the dreaded "<not accessible>" message in the locals view.
 
putItem, putValue, putItemCount seem to work ok, but as soon as I try to use Children or putSubItem, I no longer get any results. I've tried several helpers found on the Internet, for Boost types, and I have the same behavior ("<not accessible>").
 
I have Qt Creator 4.12.2, based on Qt 5.14.2 (Clang 10.0 (Apple), 64 bit).
 
Any idea what could be going wrong? Any advice about how to troubleshoot the issue (how to debug debugging helpers)?