It works fine but I don't know how to update internally read only property yet. 

Here are my changes:

1) function computing the Rect 
function updateChildrenRect(component){
    
    var children = component !== undefined ? component.children : undefined
    if ( children == undefined || children.length == 0 )
        return;
    
    var maxWidth = 0;
    var maxHeight = 0;
    var minX = children.length>0 ? children[0].x : 0;
    var minY = children.length>0 ? children[0].y : 0;
    var h=0;
    var w=0;
    
    for(var i in children){
        var child = children[i];
        
        h = child.$isUsingImplicitHeight !== 0 ? child.implicitHeight : child.height;
        w = child.$isUsingImplicitWidth !== 0 ? child.implicitWidth : child.width;
        
        maxWidth = Math.max(maxWidth, child.x + w); 
        maxHeight = Math.max(maxHeight, child.y + h);
        minX = Math.min(minX, child.x);
        minY = Math.min(minX, child.y);
    }
    
    component.childrenRect.x = minX;
    component.childrenRect.y = minY;
    component.childrenRect.width = maxWidth;
    component.childrenRect.height = maxHeight; 
}

2) above to be called from  updateVGeometry and updateHGeometry

if (this.parent != undefined) updateChildrenRect(this.parent);

3) and a childrenRect property for QMLItem

    this.childrenRect = new QObject(this);
    createSimpleProperty("real", this.childrenRect, "x","rw");  //todo ro 
    createSimpleProperty("real", this.childrenRect, "y","rw");  // todo ro
    createSimpleProperty("real", this.childrenRect, "width","rw"); // todo ro
    createSimpleProperty("real", this.childrenRect, "height","rw"); // todo ro

Thank you for the bug report.

As this report hasn't seen any changes in 5 years or more, we ask if you can please confirm that the issue still persists.

If this bug is no longer persisting or relevant please change the status to resolved.