I start with the assumption that Trump really couldn't care less if these people stay or go. He's just not empathetic enough to get worked over people not likely to give him money. His whole tough on illegals stance was a pander to a section of the Republican base and an applause line at his rallies, not an actual policy desire.
By this point in his presidency, even an idiot could see that whatever action Trump took on the dreamers would be criticized by Congress. Kick 'em out, Trump's heartless. Let the stay, Trump is usurping authority. But my taking action and starting a countdown clock, Trump forces Congress to put up or shut up.
Even better, from Trump's point of view, he's set up a "Xanatos Gambit" (warning - link leads to TvTropes) in which no matter what Congress does, Trump wins. Congress does nothing - Trump pleases his base and Congress can't criticize. Congress acts - any bad consequences are on them and Trump gets to campaign against Congress.
Now, in truth, it's much easier to set up a Xanatos Gambit if you really don't give a damn what the outcome of an event is. Since Trump cares little about outcomes and much about being seen to win, expect more of these.