Hi Mika, and welcome to the Disney Parks Moms Panel!
This is such 
exciting news! Dinner at 
Be Our Guest Restaurant is certainly a great start for a night at 
Mickey's Not-So-Scary Halloween Party!
I have been admitted early for reservations at many places across the 
Walt Disney World Resort and even at 
Disneyland Resort, so it may happen to you, too! It's wise to remember, though, that early admission will only happen if there is availability, so it cannot be guaranteed.
While I've never experienced it at 
Be Our Guest Restaurant, I've been admitted early at
 Jiko - The Cooking Place at 
Disney's Animal Kingdom Lodge, 
Via Napoli Ristorante e Pizzeria at 
Epcot and even at 
Kona Cafe at 
Disney's Polynesian Village Resort.
To increase your chances of being admitted at Be Our Guest Restaurant before 6:20 p.m., I'd suggest checking in as early as you can, so the Cast Members know that you are there and ready to get a table as soon as one is available.
I hope this advice helps you make the most of your incredible evening at Mickey's Not-So-Scary Halloween Party, Mika! If more questions pop up before your vacation is here, please, come back to the Disney Parks Moms Panel. We will be here waiting for your next questions!
May the magic be with you, 
Julie