Is it okay to bring uninvited kids to a party?

*This is a part of the Thursday rant series, a tongue in cheek look at my life.*

Warning, this is another of a "not really a rant" posting.  This one is somewhere between an etiquette concern and overreaction to how a party invitation was addressed.  Please comment.  Seriously.

I took my oldest two peeps to a birthday party this past weekend at a kids recreation center.  The birthday invitation came with the oldest two peeps name on it, not my youngest peep.  My older two peeps are in the same class in preschool; the invitation was for everyone in the class.  I had no expectations that the youngest peep would be on the invitation.

I got a gift to take and the older peeps were excited to go.  I lined up a babysitter for the youngest peep.

I was a bit surprised when I got to the party.

Their classmates were there, along with some of their classmates' siblings.  Now I don't know the full story; maybe the other siblings were on the invitation.  Maybe those families are all good friends and were not.  (not crying here -- I'm a big girl.)  Personally, I just wasn't going to bring my child to a party he wasn't specifically invited to.  I'm sure the family hosting the party had to pay on a per person basis for everyone in attendance and I just didn't think it was appropriate to save me the cost of a babysitter and bring baby peep.

Am I overreacting?  Do you ever ask to bring the other child?  Do you ask and offer to pay the difference?  Or do you just assume it's okay to bring extras?