Using the extraordinary (in my humble opinion ) geonames webservices I have found one interesting example.
I was looking for a Polish city Radom and then checked its hierarchy using the hierarchy webservice.
The immediate parent of Radom (geonameId 76077 is Masovian Voivodeship (geonameId 858787).
When I tried to list all of the cities which are children of the 858787 node (using the children webservice) I was astonished with the fact that the only child of this node is Warsaw (geonameId 6695624) ...
Then I also tried another city which should be under the Masovian Voivodeship node - Warka (geonameId 756151) and again it has 858787 as its immediate parent but is not mentioned as a child of 858787
I'm just writing here to let you know about the problem.
The children service is focusing on administrative divisions. So the children of a country are the ADM1, the children of ADM1 are the ADM2 and so on. The cities, neighourhoods etc are at the end of the hierarchy.
The problem in the examples you have found is that we don't have the ADM2 information of these cities. Theoretically we could list all PPL without ADM2 code under a ADM1, but it would be a huge list of hundreds of records.