For a clear description of the WHY of Trump’s cruelty to immigrant children please see the following piece from the Rachel Maddow Show:
http://www.msnbc.com/rachel-maddow/watch/trump-cruel-to-migrant-kids-to-drive-immigration-political-wedge-1259660867952?v=b
The entire piece is 21 minutes and 31 seconds long; however, if you are short on time see the part from 10:34 to 18:30. I could not agree more with Rachel.