Obviously, tipping is a very personal choice and is widely debated on every board I have ever visited.
Depending on the type of voucher your have (Quick and Casual Lunch/Dinner, Vacation Kingdom, etc), the vouchers will state what is covered (appetizer, entree, beverage, etc). It should also state that taxes and gratuities are included.
The vouchers will only cover the tip based on what was ordered with the vouchers. If you order three additional childs meal on a separate check, then it would be appropriate to add an additional tip to reflect the extra service received.
Eg. - You are dining at Beaches and Cream and are using 2 Q&C vouchers. The adults each order a Caesar Salad ($5.59), Grilled Prime Rib Sub ($10.99), Root Beer Float ($3.99), and share the KItchen Sink ($21.99) for dessert. That works out to be about a $67.00 tab ,with an additional $10.00 (15% tip) bringing the total to $77.00. If it was just the two of you, you could sign your vouchers and just waddle away at this point.
But the three children have each ordered a Mini Cheese Burger ($5.59) and a Coke ($1.99). That works out to be another $24.00 with the tip working out to be about $3.60.
This is where the personal decision part comes in. Some people will tip more than $3.60 on the extras. It be $5.00, $10.00, or $20.00 depending on the service rendered. That is because there is such a tremendous savings in using vouchers that it feels
funny just leaving $3.60 for what is actually close to a $100.00 check.
However, in actuality, the total tip the wait staff receives is $13.60. $10.00 compliments of WDW and $3.60 from your pocket.
I hope this helps, Steve
