The island of "Île Bizard" (GeoNameID: 5980072) does not appear as a direct child of Quebec, nor do any of the other of hundreds of islands that refer to Quebec as their parent (in their own local Geoname tree information).
Is this a mistake? Why don't the islands appear as children, even though they refer to Quebec as their parent?
You are right the documentation was misleading. I have updated it. The children service only returns admin divisions and populated places. Other feature classes are not included. The service would be become unhandy if all feature classes where included. If you need all feature classes you can use the search service.