Usually under the 14th amendment (as long as it has been understood anyway) birthright citizenship has been guaranteed to babies born on U.S soil. Typically, even when one or both parents may not actually be American themselves or birthed in the country.
According to The Hill, Trump is making it his final destiny to sign a series of executive orders that are directly aimed at harming people more than he already has. Sources told the political magazine that Trump intends to sign the EO for that directive within days if not weeks time as he is only in office until January. The directive is set to empower ICE to deport even more people and their children.
A legal showdown would almost immediately follow considering this is sort of guaranteed by the 14th amendment, however, the courts have never actually ruled on the matter.