curl -X PATCH https://api.tracklysms.com/api/v2/accounts/children/1042 \
  -H "X-Api-Key: trk_your_parent_key" \
  -H "Content-Type: application/json" \
  -d '{ "name": "Acme West (Downtown)", "externalIds": { "businessId": "b_9" } }'

{
  "child": {
    "accountId": 1042,
    "name": "Acme West (Downtown)",
    "status": "active",
    "plan": "paid",
    "externalIds": { "partnerId": "p_1", "businessId": "b_9", "locationId": "loc_west" },
    "billedByParent": true,
    "createdAt": "2026-07-26T14:03:11.123456"
  }
}

{ "error": "This external ID is already set and cannot be changed", "code": "external_id_immutable" }