May not be the the case. In the United States v. Wong Kim Ark it was ruled that persons born to persons that are not citizens but are domiciled and working in the country legally (legal residents) are citizens of the US under the 14th amendment. An executive order from a president cannot over turn a Supreme Court ruling. That case would cover any children born here while there parents are or were green card holders. The supreme court has never really held a case where it was a child of some one here illegally so this is here the lines get blurry as the present idea of jus soli has not really been challenged for over 100 years. Trump can technically define his policy on "and subject to the jurisdiction thereof" of the 14th amendment to say that people here illegally are not completely subject to the jurisdiction i.e they can't get picked for jury duty or drafted in the military service thus their children cannot be born citizens. This ,of course, will be suspended and challenged in court and we'll just have to see how it plays out there. I also would not think that the order would be ex post facto as that would be a logistic nightmare.