THE Mike, you aren't playing the "prick" card, you are playing the "smart, civilized" card.
I used to manage a movie theatre and lineups for "event" movies were always the worst. As morgardee mentioned, people would show up at the last minute and cut in with friends. I remember one woman said I ruined her daughter's birthday because I wouldn't let 10 of them cut in with 1 friend in line. For some reason people become a-holes in these situations and civility goes out the window in favour of "I want what I want."
PLEASE, PLEASE keep the "friend is holding my spot" crap from happening. I know you are going to be going in about 1000 different directions so I wish you all the luck.
I'm sure most people will understand that there needs to be STRICT supervision and no opportunities for "antics." However we all know there is almost always 1 person who doesn't get it.
A suggestion might be someone just talking to the people every 10 minutes or so explaining the rules & consequences. I know you'll be making announcements constantly, but I thought I might offer up a suggestion rather than just opining.