We love kids, so bring them if you must. But honestly, you should take the day off, have adult fun, and find a babysitter! It'll be fun, I swear! If the kiddos tag along, please make sure kids under the age of 16 are supervised (contract rules).
It's somewhere in between "Formal" and "Casual." Just look nice. Don't show up in just 'jeans and a t-shirt' or in a 'dress your mother wouldn't want to see you in.'
Did you RSVP your guest(s)? If so, then yes. If not, then probably not.