A diplomat's child is not a citizen of their country?

Yes.

State Department document (PDF).

Relevant quote:

No, the law implemented an amendment.

At the time, and ever since, the “subject to the jurisdiction thereof” bit was understood to specifically exclude the children of accredited diplomats.

14th Amendment (in part):

All persons born or naturalized in the United States and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside.

I am missing the exception for diplomats.

“Subject to the jurisdiction thereof.”

ISTM that the law expanded on what the amendment already said: Granting citizenship to those born in the US and subject to jurisdiction of the US. The law proceeded to delineate who was NOT subject to the jurisdiction of the US. Happens all the time, AIUI.

And I suspect the SCOTUS has been fine with it for quite some time, or we’d have heard about it.

That is in the law somewhere?

There it is, since by the plain reading of that phrase and the plain meaning of the definition of diplomatic immunity, a diplomat’s child is not “subject to the jurisdiction thereof.”

A child of a diplomat is not immune from local laws. Can a 25 year-old child of a diplomat flout local laws? Is a baby as immune to local law as its parent? The parent is the diplomat. Not his/her progeny.

This is Factual Questions. The factual answers have been provided to you. Under the Fourteenth Amendment, U.S. birthright citizenship does not and never has applied to the children of diplomats. If you want to argue about it, might I suggest opening a thread in Great Debates?

gnoitall covered all that in Post #20.

Has the Supreme Court ruled on this?

Is a child of a diplomat accorded the same protection from breaking local laws as their parent is?

See, for example, United States v. Wong Kim Ark , 169 U.S. 649 (1898), an exception that proves the rule. In part, the decision reads,

Again, see @gnoitall’s Post #20.

Also, largely irrelevant. Since the parents are not subject to the jurisdiction of the United States, their child does not gain birthright citizenship.

Why would the SC have to rule on whether or not Diplomat family members enjoy Diplomatic Immunity? The State Department says they do, so they do.

If you think “subject to the jurisdiction thereof” doesn’t apply to diplomats, who do you think it would apply to?

Anecdote:

My FIL was lifelong military and, for a while at the end of his career, worked in Korea for the State Department and held diplomatic status. He wasn’t the ambassador or anything, but he and his wife and their children held diplomatic passports and had diplomatic immunity. My (now-)wife used this once or twice to stop her bags from being searched at the airport in Seoul - she wasn’t carrying anything illegal or even remotely interesting as far as I know, but her status exempted her from such luggage searches.

The flipside of this arrangement is that any bad actions by the family members of the person with diplomatic status are black marks against the diplomat him/herself, and if bad enough can be career-limiting and/or result in persona non grata status. So while they can usually escape legal consequences in the country they’re residing in, there are still consequences of note.

If the Russian ambassador is speeding or gets into a bar fight they are immune from local laws?

Are their kids also immune from local law for doing the same? (really asking)

Yes, and yes. Also, yes, and yes, and yes, and yes.

Here, an American diplomat’s wife killed a British teenager and couldn’t be prosecuted:

Yes to the first question. To the second, maybe; the usual test is whether the person is a family member in the diplomat’s household. That’s why the wife of a US diplomat could claim immunity when she ran over and killed a person in the UK. If the child was not a member of the household - e.g. an adult - that weakens the argument.

I’m not sure what this has to do with whether the diplomat’s child is a CITIZEN. That is a totally different concept. This diplomatic immunity thing is not on topic. You seem to be confusing two completely different legal arguments.

Yes.

If they are under age 21 (or 23 if they are a full-time college student), yes. (Edit: and, as per @RickJay’s post, if they are still a member of their parent’s household.)

They’re not exactly immune, but under international law, as formalized under the Vienna Convention on Diplomatic Relations and the U.S. law cited by gnoitall upthread, they’re largely shielded from legal action. The U.S. can declare them persona non grata, and expel them. The U.S. can also request their home government to revoke their credentials and diplomatic immunity and allow them to be tried, which has happened in some exceptional cases. And, of course, particularly for serious crimes, their home government might recall them and try them itself.

But, yeah, google “UN New York parking tickets” some time for the decades long drama on the intersection of diplomatic immunity and local laws and ordinances.

ETA: ninja’d by a whole squad with diplomatic immunity!

In theory, the Supreme Court could rule on such a law (or on the laws cited earlier, that children born in the U.S. to foreign nationals who are in the U.S. on diplomatic/official business, do not gain U.S. citizenship on birth).

But, as I understand it, it would have to be a situation in which someone filed suit about one of those laws (such as a person who wanted to claim U.S. citizenship), and the case would have to make its way up the court system, to the point where the Supreme Court chose to examine the case and rule on the the constitutionality of the law in question.