We took her to her first game in August 2010 and she was all over the place and just wanted to look at stuff. She was about a year and a half and my wife and I traded every half inning taking her somewhere. We stayed for an hour and a half or so and that was about all she could take. She wasn't fussy or anything, just a typical toddler and wanted to look at everything.
We also took her about a month ago to dollar hot dog night against the Twins and explained to her she needed to sit down and we weren't going to get up and down with her all night. For the most part she was really good and stayed seated and we got to halfway watch the game this time. We had to make a few trips to the bathroom, which were probably more a case of her wanting to get up than anything else.
We just let her know up front what our expectations are.