It could not be retroactive. There is no question that current law makes illegal alien babies citizen. You can't take citizenship from someone who is born with it, who did not, for example, lie when applying for it. That would be an ex post facto law, and a law that takes away existing rights without due process. It would take a Constitutional amendment to do that, and you could never get an amendment passed that stripped a bunch of children of their citizenship.
Now, what you could do is pass a law that says we don't give a crap about the parents of anchor babies, they have to be shipped back. When they go back, they will then take their anchors with them. Right now, they sometimes let parents stay because their kids are citizens.