Hi, Rebecca!
Great question!
One of my very favorite sources of information about traveling with
Little Ones at Disney Theme Parks is right on Walt Disney World's website.
There, you'll find information such as appropriate attractions for younger Guests, Rider Switch (so both parents can enjoy an attraction with your older girl, without having to wait in line twice), Baby Care Centers and additional tips, too. Pour over what you see there and send more questions as you think of them; it's a great place to start!
Regarding the
Disney Dining Plan, my family absolutely loves it. We like that we have a set expense for our dining, which takes away any surprise bill at the end of our vacation. Plus, I like to know that I can order any entree or dessert that's on the menu, not having to be concerned about its cost...it's included in my Dining Plan! The DP makes us sit down for an hour each day, too, which forces us to take a break and enjoy a wonderful meal.
However, you'll have to consider how you envision your family's typical day on your upcoming trip, to see if it's worth it for you. Your younger daughter is too young for a Dining Plan, so she is welcome to eat from your plates, or you can bring some food just for her into the restaurant as well (Cheerios, teething biscuits, etc.) So, consider how the rest of you like to eat, take a look at some menus and add up what you likely spend without the DP. Compare that to the cost of adding the DP to your vacation, and you'll have your answer. (Again, we've become foodies since enjoying the Dining Plan, so my answer comes from quite a few years of comparing those costs.)
Have fun planning, Rebecca, and be sure to send us more questions; we love to help!