No shorts, jeans, or sandals. Business suits are not required, but show you mean business. If you don't own a suit, nice slacks and a shirt are appropriate. Make sure clothes are clean and wrinkle free. If you must attend the job fair right after work, take a few minutes to clean up your act in the restroom before entering the exhibition hall.
Many applicants bring another set of clothes and change before entering the recruiting area.
Leave the kids at home. If you are serious about finding a job, find a babysitter first. Leave your significant other at home too. There were several couples walking around the job fair last weekend holding hands. This isn't a date, it is a job interview.