To the OP, we had a similar situation. We went to MNSSHP 2 years ago and my older DD (then age 6) was kind of disappointed that she missed the school festivities.
We went to MNSSHP Halloween night. Yes, it was crowded walking around, but we knew that it would be crowded going in. We were prepared. We were just so excited to be at MK for Halloween!
We got our photo done ASAP right at 7:00p with no problems...we were first in line.
The trick or treat lines did appear long BUT they went SO fast that it wasn't ever a problem for us no matter where we went and we did the trick or treating in the beginning until the fireworks at 9:15p. Well, then my poor DD got sick! Throwing up all over her costume in Fantasyland while the fireworks were going.
Because of her getting sick she begged me to try out MNSSHP again so she can experience it not sick. We did
MVMCP last year and are doing MNSSHP this year again. I planned in such a way that she won't miss anything going on in school. We'll leave for Disney Oct 30 and do MNSSHP Oct 31. Now she can be at school that week before Halloween and be at MK for Halloween!
I think Halloween night can be very busy but it just depends where you are in the park and what you are trying to do. I say get the photo over with, wait for the second parade, the trick or treat lines are not as long as they appear because they go VERY fast, head to the back lands and I'll bet it's not as crowded. Relax and take in all the decorations and music and atmosphere. It's GREAT fun!
