@Nancywanett
The museum is open from 10am to 6pm...
If I was trying to avoid traffic, I would plan to arrive by 10am... should take you about 40 minutes if you leave by 9:15am
If you leave the museum district by 4pm, it should take you 40-45 minutes to return to the Big A
If you leave at 6pm, estimate 1 hr...
Since you have more than one passenger, you can get on the 59 North HOV for free (after downtown and before 610)... that should save you quite a bit of time if you happen to be near that area after 4:30