Hi Rachel,
It really depends on where you are dining! There's a significant difference between a meal at Beaches and Cream vs. California Grill, for example. Plus, it'll depend on whether you are ordering the most expensive entrees/desserts. Remember, pre-paid restaurants and dinner shows automatically add in the gratuity.
I would take a look at restaurant menus, then estimate your bill. You'll want to figure 18-20% for the gratuity.
Let's take an example:
dinner at Sci-Fi Dine-In Theater
Here's the
menu. Of course your bill before tax will be lower if your son is age 3-9. In that case, look at the
Kids Menu.
On average, your plan to allot up to $15 in your budget per meal for tips should work well depending on your chosen restaurants. However I would do some quick calculations based on what you think you might order by checking out the menus of the places in which you have made your Advance Dining Reservations.
Have a wonderful time in WDW!