I am going to have to say no, it is rude for you to expect parents of 5 year olds to drop them off at an outside birthday party, and if they stay, to expect them to pay admission.
Let me explain where I'm coming from:
I have a 5 yo daughter and a 6 yo daughter. I did drop off parties for both of them when they turned 5 and 6. These were at my house. My 5 yo had 8 girls over for a PJ party, (not sleepover) and parents dropped off at 5:30 and picked up at 8. My 6 yo had a pool party (3 foot high above ground) and we had our babysitter stationed as a lifeguard at the pool. This was also a drop off - 3:30 to 6 but some parents wanted to stay. I had enough food but nothing fancy since it was a kid party.
Alot of our friends do parties at outside places and always order extra food for adults. In the years we have been going to parties, there has only been one time we went to one with no food for adults, it was a Mcdonalds birthday party. I was not at all offended though. Just a comment. At the outside places (karate, gymnastics, bowling, dance, laser tag, pump it up, etc etc) parents NEVER have to pay. They are basically coffee clotching and keeping an eye on their kid.
At the home birthday parties my kids have been to, the parents go nuts with food and booze for adults. I don't think anyone has to do that!
I am pretty much the only one who has started "drop off parties" because my house gets too crowded!
Anyways, I would not expect a movie theater drop off party until at least 7 years old. That's my opinion.
I would absolutely say "no siblings please". But I think parents bringing siblings unless SPECIFICALLY invited, are very rude. And I have 3 kids, and a husband who works alot, so believe me, I understand the juggle involved here. I either have my other 2 dropped off with a friend or relative or ask another friend who's attending the party to take my kid.
In your case, since it is already planned as a movie party - I think you should state on the invite one parent per child is included as a chaperon, and you should ask the theater to have them attend for free. It's silly for them to have to pay.