You cannot win on the optics of sending 'children' back to a country where they may not even speak the language. It doesn't matter if you argue until you're blue in the face that these aren't children, but rather 20-30 somethings. All it takes is a good photo journalist to frame a few good shots (tearful separation, dead child, starving waifs) - staged or not (most likely staged) and it's all over except for the crying.
Knowing he can't win, Trump can sacrifice a pawn - or even knight - to achieve a few key objectives. The wall needs to be funded/built; tax reform needs to be done; and O-care has to be repealed.
Whether these objectives are accomplished with existing Congress, or the issues are used to bludgeon all incumbents leading up to Nov, 2018 doesn't really matter. What matters is fulfilling broad, strategic goals.
You want to circle the ones we should keep in this photo?