What should we say to affected students about Trump's travel ban?
Put it in its historical context.

The United States can be a wonderful place to visit or even live, but it does have a long term history of periods of political unreasonableness. During those periods, it becomes so afraid of some group of "others" that the fear overrides human rights, treaties, justice, common sense, and the US constitution. Keeping the US safe from the feared group is the overriding concern, and nothing else matters compared to that. Often, the feared group includes a few people who really are dangerous, but the danger gets grossly exaggerated and makes the US deeply afraid of an easily identified larger group.

There was a mild one during Nixon's presidency, when the feared group was people protesting the Vietnam War. The most recent really severe one, in the 1950's, was McCarthyism, when the feared group was communists. In WWII the feared group was people of Japanese origin or descent. Now, the feared group is Muslims, and people stereotyped, based on e.g. national origin, as being Muslims.

The good news is that, after a period of fear-based unreasonableness, the US always sorts itself out and gets back to sanity.

Any students from one of the affected countries, and perhaps Muslims in general, who need to make immediate decisions about e.g. a postdoc should consider going somewhere other than the US. If you are planning an international conference, it may be better to pick a country whose visa policies make it more likely that all interested academics can attend.

In the longer term, the current fear of Muslims will go the way of fear of Irish Catholics, fear of Chinese immigrants, Japanese-American internment, and the McCarthy era.