@Garrett seems like AAA has already stated a lot of the basics, but I'll echo the most important part: you can't do everything, don't try, don't push it, don't even. It will only lead to cranky families and the "I'm never coming back" mindset. WDW will always be there. Take your 1st trip, relax, soak it in and if you enjoy it then start planning your next trip.
according to the crowd prediction calendars, you picked a good week. Its a little up and down but most predictions are in the 5-6 range (out of 10 being the most crowded) for the parks. So thats good for getting slightly more accomplished then those days in the 8-9 range.
The resort you picked is a really good choice. Yes, it is deluxe, so you will be spending more per night then other resorts (if you want a breakdown of the resort categories and benefits, just ask), but the location can't be beat. With the kids ages, the MK will be your primary park and you'll have both monorail and ferry boat access to there which saves a lot of time. My only other resort choice would be the contemporary. It lacks the theming of the polly, but it is the only resort where you can actually walk to the MK and if you are going to Epcot via monorail, its quicker to the transfer then the other b/c it is the very next stop from the resort. But either way, the location is paramount and you are in a good place.
the dining plan is always a hot topic. Dining at Disney can be considered an attraction in itself. Not that the food is overly amazing, but most of the restaurants are themed very well and there are the character meals that are so very much part of the WDW experience. And while generally i am a big fan of the dining plan, I'm not sure if it is worth it for you for this trip...that is depending on how your family eats. If you enjoy sit down dinners and don't mind taking some park time away from your day for a meal, then that may change things. The good news is that you don't need to decide on that until you are very close to your trip, so lets not get lost in that now. I would investigate the cost and benefits and keep it as a part of your budget. Dining plan or not, you will need to spend money on food regardless, so use that cost as a benchmark—either out of pocket or with the plan.
While I've never used a travel agent, don't discount their usefulness based on the experiences you had. One of their main jobs is to keep checking for discount for you, even those not announced to the public. Plus an official WDW travel agent will not cost you a dime, they get paid by disney as part of your booking. So even if they find you a $1 in savings, they paid for themselves. Plus they might have an slight inside track on hard to get dining renovations or other ins. Some will provide additional value, as in i met a family on the plane once who's agent made them a special flip book of all the attractions and other strategies for their trip. Its worth talking to them if you are really unsure of the real scope of a WDW vacation. I have a few i can recommend if you are interested.
Lastly, you will be there during one of the best times of year at epcot—Food and wine festival. Google it for the best way to understand it. But when thinking of budgeting your park time, give Epcot a little extra hours b/c IMO its the best thing that that park has. Esp if you are slightly adventurous about food and don't mind sampling international foods. Also, events like this, really muck up the dining plan debate, bc I was stuffed when I went and couldn't even consider a sit down meal that day.
I'll shut up now, but feel free to ask questions as you work your way through the process. Just keep certain dates in mind once you book:
•180 days from check in you can make dining reservations
• somewhere around 80 days, make sure you have your magical express luggage tags.
• 60 days make your fast pass selections
if none of the makes sense to you, feel free to ask...it can be an overwhelming vacation once you hear vets talking about stuff. lol
