Welcome to the Moms Panel! Thanks for coming to us with your question.
Tips are not included on the Disney Dining Plan. When you receive your check at the end of the meal, it will list everything you ordered, and put a total of what the bill would be without the Dining Plan, then will put your total of $0 (unless you added on items not included in your plan). You can then add on your tip based on what the total would have been (customary tips are around 15 - 20%) and charge this with your credit card, charge to your Key to the World card, use cash, or even a Disney Gift Card.
The 18% is included if your party is 6 or more guests, but you still need to sign for the tip. Click
HERE to read more about the Disney Dining Plan.
I hope this helps! Have a great time at Port Orleans French Quarter (one of my favorite resorts!)