Okay, I'm one of those parents that brings a sibling. My dh is an airline pilot and often out of town, and neither daughter is old enough to stay at home alone. Now having said that I will add that I always call the bithday child's parents in advance and ask if I may bring my other daughter always stating that I will pay for her admission seperately. Only once was I told that I couldn't, so we didn't go. Now, that we have been in town for a few years and have people we trust to watch our girls I would try to arrange a playdate if she couldn't attend. However, I wouldn't pay for a babysitter. I have also had others do the same to me. I've had people bring siblings, and I've had people just show up without RSVPing, but I've never had people bring siblings without RSVPing.
About schools, it is policy in our school district that if party invitations are handed out at school, every child in the class MUST receive one. Otherwise, we are to use the school's directory and mail the invites.
So, OP..... what did you decide to do? I, too, liked the movie mom's wording, and I also agree with the others that say to pay for 1 adult. At that age, I would be uncomfortable leaving my child with people I don't know very well or even at all.