Hi Michael,
When searching for a table at
Be Our Guest restaurant on the Walt Disney World website, simply enter the date when you would like to make a dining reservation for, select "Dinner" from the "Time" drop down box and enter your desired "Party Size". (So, assuming that you have 2 daughters, this would be a total of 3 including your wife.)
I just attempted to make a dining reservation for Dinner for 3 people on August 29, 2016 (which I think is your arrival date at the Walt Disney World Resort) and a 8:45 PM dining reservation is currently showing as being available to book.
Once you start to go through the confirmation process, you will be asked "Who Will Manage The Reservation" and you are able to select anybody from your Family & Friends list on My Disney Experience.
As you yourself won't be attending the dining reservation, I would suggest that you select your wife. You can then go on to choose your daughters as Guest 2 and Guest 3 of the Dining Party.
Wishing you a magical trip filled with memories which will last a lifetime!
Emma