In all honesty with this being your first MNSSHP I'd recommend eating before you arrive at Magic Kingdom and arrive at the entrance between 3:15/3:30. Just pick up quick serve if you get hungry. Disney has set up mobile ordering which is a game changer as it allows you more time to actually enjoy the Halloween offerings without feeling rushed or waiting in long lines for food.
This will be my first time reserving the dessert party. I really recommend it for guests who are looking to enhance their experience after many years of attending this event as well as those who may need it due to disabilities or any other physical issues.
Touring Plans has your date ranked #4 for rides so you should be able to breeze in & out of those.
https://blog.touringplans.com/2018/05/30/least-crowded-mickeys-not-so-scary-halloween-party-of-2018/
Our buddy and fellow DISer Kenny the Pirate has overall crowd capacity at Moderate which means you should not have a hard time finding spots for the parade nor the fireworks.
http://www.characterlocator.com/?p=cal_index&tdate=09/23/2018
With just 2 adult tickets and 1 child (age 8) = $232
MK 4pm to 12:30am = 8.5hrs. of party time
You're paying $27.29 per hour and this doesn't include tax.