We've been the last two years during Halloween, and are very excited to be going again.
As for advice.....hmmm, probably just the usual things like go early and during the week to avoid crowds. October used to be considered a slow time but not anymore. It gets packed! This is a pic I snapped on a Saturday in front of the Haunted Mansion last year. Shoulder to shoulder people and strollers everywhere. But during the week, it wasn't bad.
Last year they had Jack and Sally out posing for pics and signing autographs. Get in line early if you want to meet them. They close the line off pretty quickly and I couldn't understand why at first, until I saw just how long they took with each guest. The interaction with them was really fun, and lingering.
There are lots of special Halloween themed treats offered during this time so save room in the budget and stomach for those!

Especially the famous pumpkin muffin which is really good.
They even have special jack-o-lantern shaped sourdough bread for purchase over at the Pacific Wharf Cafe.
I'd recommend taking time to really take in all the decorations. The windows and shops walking down Main Street are all decked out. Even the horses get into the act and the napkins are themed! It's those little touches everywhere that really immerse you into the spirit of the holiday. (Yes, I actually took a pic of the horse's behind and a napkin. LOL But what can I say, I was impressed that Disney thought of everything!)
There is lots of fun Halloween merchandise for sale. And if your girls are into the characters, you'll have plenty of photo ops to take pics of them with the characters in their Halloween costumes.
Last year, Minnie was a witch, Goofy was in a skeleton costume, Chip and Dale were a cop and robber.
Near the Mexican restaurant and Big Thunder Mountain, they have a Day of the Dead display with some artists offering free "tattoos". Many people were walking right by because they didn't realize it was free to do. So we stopped and let the kids get it done. After all, how many things in Disney are free! We had to take advantage of that. lol
Okay, well, I'll shut up now. It's just a really fun time to be in the parks and I can't say enough about it. Hope some of this was useful. Oh, and I love your Mickey Pumpkin Shirt.