Convert parent doc to child doc in 6.6

In a 6.6 index, I have a join field 'relation' like so:

"relation": { 
  "type": "join",
  "relations": {
    "parent": "child" 
}

Several hundred parent docs need to become children to another existing parent doc. I have lists of all the relevant _id values for 'demoted' docs and 'winner' docs. For example, I need to make 5678 a child of 5677

{
  "_index": "myindex",
  "_type": "place",
  "_id": "5678",
  "_source": {
    "children": [],
    "relation": {
      "name": "parent"
    }
  }
},
{
  "_index": "myindex",
  "_type": "place",
  "_id": "5677",
  "_source": {
    "children": [],
    "relation": {
      "name": "parent"
    }
  }
}

Currently, all child records include "_routing": "1". I don't remember why I configured it that way, e.g.

{
  "_index": "myindex",
  "_type": "place",
  "_id": "6368962",
  "_routing": "1",
  "_source": {
    "children": [],
    "relation": {
      "parent": "13005543",
      "name": "child"
    }
  }
}

So the goal for the demoted doc is...

{
  "_index": "myindex",
  "_type": "place",
  "_id": "5678",
  "_routing": "1",
  "_source": {
    "children": [],
    "relation": {
      "parent": "5677",
      "name": "child"
    }
  }
}

I have tried this _update in Kibana console

POST myindex/place/5678/_update
{"script":{
	"source": """
		ctx._source.relation=params.relation;
		ctx._source.children=params.children;""",
	"lang": "painless",
	"params":{
		"relation": {"name":"child","parent":"5677"},
		"children": [] }
}}

and get this error:
"[routing] is missing for join field [relation]"

I tried adding ctx._routing=1; to the script, and it has no effect.
Is there no way to "demote" a parent to a child and specify its new parent?

thx