K.A.
I think what would pass for tacky 20 years ago, might be considered acceptable now, because I think, as a society, 20 years ago, we would've never had parents bringing extra kids, or not responding to an invite. Things are different now and people have gotten lax in their common sense and manners. I would hope that parents knew that only the invited child was paid for. Since some don't, you should probably spell it out for them, nicely, on the invite.