How to dump a dict to a JSON file?

I have a dict like this:

sample = {'ObjectInterpolator': 1629,  'PointInterpolator': 1675, 'RectangleInterpolator': 2042}

I can’t figure out how to dump the dict to a JSON file as showed below:

{      
    "name": "interpolator",
    "children": [
      {"name": "ObjectInterpolator", "size": 1629},
      {"name": "PointInterpolator", "size": 1675},
      {"name": "RectangleInterpolator", "size": 2042}
     ]
}

Is there a pythonic way to do this?

You may guess that I want to generate a d3 treemap.