Legal Question in Constitutional Law in Texas

Your question seems to presume that legislation can prevent "anchor babies" from becoming citizens, but that's not true. Their citizenship comes from the Fourteenth Amendment. American-born children (except those born to diplomats and others who are here conducting official business of their home countries) are constitutionally entitled to citizenship. No legislation can change that.

Even if this were a legislative matter, then new legislation could only apply to children born here in the future. It could not strip current citizens of their citizenship.

And yes, even laws that have never been tested in court are subject to the ban on ex post facto changes.

An ex post facto change in the Constitution would be constitutional by definition. The issue has never come up so I don't really know what the courts would say, but it is hard to imagine that they would invalidate a new provision because it conflicts with an old one. After all, several amendments were intended to modify existing provisions; if such a conflict could invalidate the new amendment then there would be no way to modify anything the Constitution already says.

But an amendment which stripped people of their citizenship would be manifestly unjust -- especially if it did so based upon circumstances over which they never had any control, like the place of their birth. It is hard to imagine such an amendment being approved by two-thirds of the House, two-thirds of the Senate and three-quarters of the states, which is what amending the constitution requires.