Actually many get a drink as soon as they board as a 'start vacation' celebration. And you don't need to use your own Sign and sail card. once person can buy all the drinks. I cruise a few times a year and our first stop is atrium bar to get that first 'cheers' drink. Its tradition.
That and from the timeline from Winkleman's filing we know that the family had lunch after boarding just prior to mom and Chloe going off to play in the splash pad. It's quite possible that a cocktail or two was enjoyed with lunch.