Our first trip was sometime in the early 80's with our kids (youngest in kindergarten then) and we are now long time membrs of DVC and returning several times a year from PA with those kids, new in-laws and two granddaughters! It is good to get some info on what you think you want to see and start with a "flexible" plan. You may find the kids are more interested in some things, not interested in things you planned a whole day to do. Epcot isn't really a whole day of interest for younger kids. The weather, the activity level, the lines etc will alter alot of "plans". Remember, this is a VACATION - for the kids too! Go with the flow and have a ball - you will be back! Budget-wise, when our kids were young, we did not plan meals in sit down restaurants which typically cost alot more - require reservations (more plans) and kids don't want to stand in line for an hour when they are tired to get mac and cheese or a hot dog from the kids menu. The meals in the fast food in the parks are great and I think a good value. There is a HUGE Mcdonald's on property! Relax and enjoy! We are leaving for WDW in 2 hours!!!!!
. And there certainly will be things that you want to go back to see / do on another day; no need to feel rushed.
Happy Planning!
And as far as how many days do you need? I would say that you could spend 30 days there and still not see everything in one trip.