His sea pass vouchers will not get you alcohol but can be used for mocktails, fresh squeezed OJ, soda, bottled water, coffee drinks, etc. There are other differences... like he won't get a free picture or the $6 or whatever in free casino slot play.
My slightly older nieces are Diamond and became quite addicted to virgin daiquiris on our family cruise a few months ago, lol. As an uncle, it was fun being able to take the 12-year-old "to the bar."