Hi Adam,
First, let me say that you've already done something special for your wife and daughter: you've planned your first family vacation to Walt Disney World. My guess is that this will be the start of a long tradition for your family.
My suggestion to be sure you capture all the memories -- and get pictures that show the whole family -- would be to consider the
PhotoPass+ service as an extra gift to your wife and yourself. The PhotoPass+ card, which you purchase in advance for the best rate, gets you unlimited photos by the professional Disney PhotoPass photographers stationed around the park.
It's a way to be sure you get "the shot" in all the iconic spots around Walt Disney World and have lasting memories to display.
You should also be sure your wife and daughter get a free "My First Visit" button at the desk of your resort hotel or at Guest Services at the front of any theme park. Cast members and guests will offer their congratulations and, occasionally, a special surprise.
My other best piece of advice is not to stress. Go with the flow and enjoy your time as a family. You can't plan the most special moments. They will happen spontaneously, especially with an 18-month-old.
I hope that helps. Please let me know if I can answer any other questions. Enjoy!
Wilma