Which is just it. Besides the fact that certain procedures must be followed in order to turn a foreigner into a citizen, what about the people who are temporary visitors and just happened to have a child born here?
Government has no authority to claim any of that child's allegiance by making that child a citizen against the parent's wishes.
As I have pointed out ad infinitum in the past, for a Monarchy, grabbing the children of foreigners in your country is a feature not a bug. It has the practical effect of instantly creating more servants (which is what a "Subject" really is) for the King.
And we are not the first people to notice how useful such a thing is for a Monarchy, but how inappropriate it is for a Republic.