Trump is violating U.S. Federal law and International law by ordering asylum seekers to wait in Mexico. The laws VERY CLEARLY state that asylum seekers are to wait in the country they are seeking asylum in while their application is being processed. This will surely be challenged in court soon.
The Trump administration is also violating the Flores Settlement, which places rules on how the U.S. must treat immigrants and asylum seekers. There are time frames regarding how long they can be held and rules requiring the government to teach the children English and provide them with recreation.