actually it is when the parents are actually caught.
Keep in mind what happened post-1996 was that immigration lawyers shifted to a “undo hardship” to claim anchor status.
Usually it was a special medical or educational need for the minor that was not available in the old country. Usually the immigration prosecutor just showed the US foreign aid or schools proving the contrary. (adhd, therapy etc.)
This is why the Dream Act Amnesty was so vital. It created anchor babies by mere attendance at a public school. It put custody in the hands of the public school attendee. (children shall lead concept)
Enforcement is just not publicised unless a lawyer is doing it as a PR stunt or desperation measure.