Will you just do Disney or do you think you will get to other parks as well, like SeaWorld? At that age, unless they are major Harry Potter fans, Universal could be wasted on them as there are more "big rides" there than at Disney.
Sign up for a Disney account, either by getting the MDE (My Disney Experience) or through their website. Add on all the family members and when you purchase your tickets link them to your account.
I am not really sure that I would call anything a must do, you might not like the same things as me. However if you have girls, the Bibbido Bobbidi Boutique is a nice treat, followed by a princess character meal. Other character meals are nice too, Chef Mickeys, Tusker House, Garden Grill for example. The Be Our Guest Restaurant is very popular, although the food is mediocre, we always book it for 8am or early as it gets you into the park ahead of the crowds.
You will be able to book restaurants through your MDE account up to 180 days in advance. You will be able to book your fast passes 30 days in advance as an offsite guest.
I have a spreadsheet of which park I will go to on which days and I use this to know which fast passes I should book. You can book three FP in advance but they need to be in the same park. After you have used up these three FP you can start to book more one at a time and that can be in any park.
Yes I think it will be busy in October, the food festival is on in Epcot and it will be decorated for Halloween and the Halloween parties will be running. However if you make the most of your FPs you can avoid some of the queuing.