To be more precise, you must buy tickets in advance or you won't get in -- I wanted to visit the museum this weekend, but all the times that fit my schedule were already booked solid several weeks ago. FYI (or your kids' information, as I presume they're the anime consumers of your family), Ghibli is the studio that produces all of Hayao Miyazaki's films, and they have a special exhibition also at the Expo which is ever harder to get into, but there's a special quota of 40 seats/day for foreigners if you apply by mail at least one month in advance (enough bold?). See here for details.

And yes, Expo 2005 is a winner ^