The assumption of their stay being at the Palce Hotel (now Radium Palace Hotel) is the upscale look on the front of the postcard. Also look carefully at the white chairs, and the chair on the back left. They match the ones seen in these pictures which are from 1946 and apparently also come from the hotel:
http://sechtl-vosecek.ucw.cz/en/cml/35mm/film35mm4327.html
http://sechtl-vosecek.ucw.cz/en/cml/35mm/film35mm4325.html
http://sechtl-vosecek.ucw.cz/cml/35mm/film35mm3806.html
http://sechtl-vosecek.ucw.cz/en/cml/fotografie/foto3295.html
