you can become a US at birth by either being born on the soil or having at least one US citizen parent that had spent more then 5 years in the US since they were 14.
as such... if prince william and kate flew to NY and delivered their child while on vacation, the child would be a US citizen AS WELL AS the future king of england. this is EXACTLY the situation the founders wanted to insure against.
as such, they used the phrase ‘natural born citizen’ as a requirement for the president, the only position in the country with such a requirement. this would disqualify william and kate’s kid as he had multiple citizenship possibilities at birth. this same test can be applied to bh0bama, mrubio, bjindahl and tcruz.
once again, a natural born citizen is a citizen naturally... as there are no alternatives.
it’s that simple and was common parlance (at least in the US and EU) during the time of the founders.
The royal example you state is a bastardization of the Constitutional intent. If either William or Kate WAS an American citizen, the child WOULD be American, and eligible for the Presidency. Bob