Very funny line about needing your real friends to sit! Love it!
I agree you are already spending a fortune. The kids will be bouncing off walls as it is! And I think the party favor/goody bag idea is really overdone, with parents giving (no kidding) a $25 gift card to every guest when the birthday kid got a $20 present! If you buy candy/drink/popcorn, serve cake, and then send them home with another movie ticket plus M&Ms, you'll be broke and the kids will all be diabetic by morning. And Halloween is coming up too - another giant sugar rush. (Many of us in our neighborhood don't give candy for that anyway since one child is a serious diabetic. We give favors like stickers and pencils and tattoos and slap bracelets. But that's another topic!)
I love the idea of some popcorn and the note. The dollar stores also sell plastic containers that say "popcorn" - tall rectangles that resemble the movie containers. You could put the microwave popcorn in that.
Also, instead of getting each child their own combo, see if the theater will sell you a large bucket of popcorn and then let you divvy it up. We see that all the time at our theaters - your bring your own large cups (like Solo cups, maybe 9 or 10 ounces) and use one for scooping out from the big bucket into individual cups. That keeps down the mess, litter and the expense instead of each kid getting too much food that they are just wasting because none of them can finish it.
You could also buck the trend and say the kids don't need a favor for coming to what's already a $30+ per child event!!