All US citizens have to have a passport to travel internationally. Whether they are newborns or 97 years old.
Archie has US citizenship through his mother whereas Lili has it as she was born in the US. Both children have passports.
Regarding American passports - I know for a fact that I couldn't travel on my own individual one until I was about 8, Which these two of course are not. The rules may have changed, on the other hand these two are much younger. I suppose their father did apply unseen for the UK passports at some point in the past two years. Why else would they still be in the line of succession? Are non-UK descendants allowed to be in the line of succession? On the other hand, I suppose it might be so - since didn't Prince Ernst-August of Hanover have to request the Queen's permission to marry Caroline of Monaco in the 90s, and he's a German citizen? So I suppose some people find a way to "have their cake and eat it too" - in this case, finding a way to be American, but finding a way to be a "prince" and thereby qualifying for that much "deserved" police protection?
Are you sure the US does not issue passports to children? They did when my children were little in the 1980s. Their mother and I lived on opposite sides of the Atlantic at the time and they flew rather often between the two of us.
When I worked for an airline in early 2000s, I don't recall what the exact rule was but I never encountered a child international traveler who did
not have their own passport.
A question in light of the fact that the two Sussex offspring are now scheduled to travel from the U.S. to the UK: since the UK does in fact issue "Child Passports", but the U.S. does *not*, does it not also mean that: (a) When Lilli M-B was born in California, her father would have had to have registered her birth at the nearest British consulate as a "child born abroad of a UK subject"? Which in turn allows her to travel solo now with her grandmother into the UK? And then (b) One wonders how these two children have traveled on previous travel outside of the U.S., since in fact they also qualify for American citizenship? So, were they previously on Markle's passport until now, or did they always have British "child passports" of their own? All very murky, but very convenient when you want to claim a princely title for your children isn't it?
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