I know first-hand everything you are feeling planning your first trip to Walt Disney World as it can certainly be overwhelming. I'm so glad you have your sent your question our way, and please know you can send as many as you need to help with your planning!
Since you are traveling with little ones, Walt Disney World actually has a planning page for
Planning for Pre-School Kids & Little Ones which is a great starting point. You may also want to play around with the
Attractions,
Character Experiences (part of Entertainment), and
Entertainment pages where you can do a "sort" through each theme park to see which things are geared toward pre-school guests.
I'd also begin visiting the
Disney Parks Blog regularly for news and updates. I also really like www.disneyfoodblog.com for food updates and dining information. I'd like to also suggest www.touringplans.com which has a lot of helpful information, but it also gives you the option to purchase for a minimal cost an assortment of touring plans (there are specifically ones for touring with a pre-schoolers) that lead you through the must-sees at each park and when to visit them to help reduce your wait time. I know you are trying to avoid the cost of a Disney Vacation Planner, but I think you'll find this still significantly cheaper and a good investment. Even with our many trips, we still use it for every trip!
As you begin to look things over and find you have more questions (where to eat? what are good planning books? etc), feel free to send another question our way.
-Kara