This is how it plays out:
pregnant woman/family crosses the border undetected. Woman birth's kid in the US. Family remains in the US working under the table or buying an alias/fake paperwork. This continues until one of two things happen:
A. Family qualifies for some form of legal relief (U visa, marriage to a citizen, dreamers act, temporary protected status, citizen baby turns 18 and claims parents, etc). OR
5.) Mother/family member gets arrested or pulled over for whatever reason and it's discovered they are undocumented. Notice to appear in court is issued. If they don't appear a deportation order is issued. If they never arrive then they might remain undetected until they get caught/arrested again, once caught again they'll find the deportation order and they'll be detained and deported. If they did present themselves to court a deportation order will also be issued and they imediately get detained and removal proceedings start for their deportation. Since the child is a citizen the parent can chose to be deported with their child or arrange a caregiver in the US for child to remain in the country since they are citizens.
Trumps line of thinking is the following: Immigrants are willing to do anything and risk anything in order to secure their children's citizenship by birth, so let's take that right away for children of undocumented people.