Pitt, who is Canada shooting The Assassination of Jesse James, called the museum to ask what time he and Jolie might visit without attracting too much attention, according to a story in Tuesday's Calgary Sun.
After 7 p.m. on Saturday, the couple showed up with Jolie's four-year-old son, Maddox, who loved the T-Rex exhibit, said Wendy Taylor, a spokeswoman for the museum.
"They did not ask us to close the museum or for an after-hours visit or any special treatment," Taylor added.
Meanwhile, Jolie's 7-month-old daughter Zahara, who was also along for the outing, "was smiling and giggling – she was so adorable," an observer tells PEOPLE.
"The other people in the museum at the time were very respectful," Taylor said. "No one approached them on their own."
The only down side to the visit: As word of Pitt and Jolie's presence spread, a curious crowd gathered outside and the couple was forced to sneak out a different exit, Taylor said.