def __call__(level):
    """Returns children categories of CategoryContainer like:
        
        [
            {
                'item':<Products.ZCatalog.Catalog.mybrains object at ...>,
                'depth':1,
                'children':[
                    {
                        'item':<Products.ZCatalog.Catalog.mybrains object at ...>,
                        'depth':2,
                        'children':[
                            ...
                        ]
                    },
                    ...,
                ]
            },
            ...,
        ]
        """
