"Some expressed dismay that Trump, whose wife, Melania, has chosen to stay in New York to avoid moving the couple's 10-year-old son, Barron, to a new school midyear, would not ensure that such allowances be made for U.S. ambassadors."

The difference here is so obvious:  Melania and son are not political appointees of a defeated party, and they are living in their own home, not government property.  

All ambassadors knew this was a possibility for the past four years, and they have known for sure since Election Day.  Each of these families could have found other living arrangement by now had they made the effort.  It is their own fault if they haven't done so.