The big buzz after the DNC was about the speech given by... must... resist... urge... Khan!!! Damn it.
Anyway, Trump's response was, unsurprisingly, foolish. He attacked the family for the mother's supposedly compelled silence, with an insinuated but unstated reason.
One of two things must be true:
1) Trump's campaign briefed him on the possibility of this coming up, and fed him the appropriate response.
2) Trump's campaign failed to do so.
If 1, the obvious response is to evade, and simply say, "I'm sorry for the Khan family's loss, and I won't comment any further."
Since Trump didn't give that response, either his campaign didn't prepare him, demonstrating their further disorder, leaving Trump to improvise, or they tried, and Trump just couldn't resist a dig because he's Trump and that's just his schtick.
Social science buzz-word of the day: "observational equivalence." (Technically, that's two words). That's when two models predict the same outcome. Model 1: Trump's campaign is a disorganized mess. Model 2: Trump is a jerk. Observationally equivalent. See? Even when I'm messing around, I'm trying to teach social science.
And because I'm still an asshole...