Completely Lost: Many To Many with Serializers and Update in Django Rest Framework

I think your JSON is not correct. It should look like this:

{
 "id": 1,
 "url": "some url",
 "name": "John Smith",
 "children": [
   {"id": 2, "url": "child url", "name": "childs name"},
   {"id": 3, ...}
 ]
}