Let us go back to basic principles. Royalty only extends to
grandchildren of the sovereign; after that they become 'non-royals'.
Peerages of the United Kingdom, unlike those of some other countries,
extend only to the actual holder of the title, so peers' children are
all commoners, including the eldest son before he inherits the title.
Agreed. But we're talking about courtesy titles here, and they're
titles borne by commoners.
Journalists don't have time to look things up properly or to think
things through logically, and so regularly get titles wrong.
Where do you expect journalists to look it up? I've tried quite a few
of the obvious sources, and very few of them address the issue of
grandchildren of peers, other than the heir apparent of the heir
apparent who would normally get a secondary courtesy title, if one is
available.
If you look instead for example in the common reference works, you often
find errors in them, frequently due to them not being properly updated
when the holder of the substantive title dies.
Debrett's, which is generally well-regarded (perhaps more than it really
deserves), is a good example of that. Look up Princess Margaret's
granddaughter, Margarita Armstrong-Jones. As the daughter of an earl
(and not just one by courtesy) she unquestionably gets the title "Lady",
but Debrett's labels her "The Hon." instead.
Any recent changes to the use of courtesy titles will have been made
under an Earl Marshal's warrant or (less likely) letters patent, but
these are almost impossible to locate. In any case, most of these
styles has not been formally defined and is simply a matter of custom.
We should not be misled by their mistakes. Instead, apply the principles
above: peerages apply only to the holder of the title, and courtesy
titles, if applied, only last for one generation.
Except that's definitely not true as there are some very clear cases
when grandsons gain a courtesy title. Lord Culloden (the eldest son of
the eldest son of the Duke of Gloucester) and Lord Downpatrick (the
eldest son of the eldest son of the Duke of Kent) are examples.
It's also pretty clear that the daughters of an earl by courtesy are all
styled Lady, just as they would be for a substantive earl. For example,
Lord Downpatrick's sister is consistently styled Lady Marina Windsor
when she appears in the daily Court Circular released by Buckingham
Palace. I think repeated use in the Court Circular can be considered to
make the usage correct by precedence, regardless of what rules may exist
on the subject.
Similarly, children of viscounts by courtesy are styled Hon. An example
is the Princess Margaret's grandson, the current Viscount Linley.
Before the death of his grandfather, the first Earl of Snowdon, he was
consistently styled The Hon. Charles Armstrong-Jones in the Court
Circular, just as he would have been if his father was a proper viscount
rather than one by courtesy.
I think these examples make it very clear that the children of the
eldest son of a peer can frequently end up with a courtesy title.
This really only leaves the question of how the children of the younger
son of a duke or marquess are styled. The younger son will be styled
"Lord", but does that mean his children get the style "The Hon."? If I
had to guess I'd say no, but it's not clear what principle if any we
should be applying. Amongst the aristocracy (as opposed to, say, Law
Lords or Lord Mayors), I can think of no other circumstances in which
the children of someone using the style Lord (whether by courtesy or
otherwise) would not be styled "The Hon" or higher.
I cannot see any source addressing this question in the general case, so
the next best thing is to look for a definitive reference on a specific
case. Within the descendants of Edward VII, I can only find one set of
examples: the three sons of Lord Nicholas Windsor, who is a younger son
of the Duke of Kent. But so far as I can see, they have never appeared
in the Court Circular and I don't think Buckingham Palace ever announced
their births. They're listed in line of succession given in Whitaker's
Almanack as "The Hon. Albert Windsor", and they appear in Debrett's line
of succession without "The Hon.", but as noted above, I wouldn't
necessarily trust either of these sources.
Richard