JimMIA hit the nail on the head. How you tip is up to you, just let your server know what you plan on doing.
To clarify the mandatory tipping confusion for you:
All bills will show what an 18% and what a 20% tip would be - how much you leave is up to you. There are two exceptions to this.
Party size is 6 or more. Does not matter what the ages are in the party, nor does it matter if you are two groups of under 6 who are dining together. 6 bodies at a table = 18% mandatory gratuity for all parties at the table.
You are using the DDE (Disney Dining Experience Card) in this case, no matter what your party size is, your group will have to pay a minimum of 18% gratuity.