I believe "Tea with..." (Alice, Princesses, etc.) is offered only on the classic ships (Magic and Wonder) on 7-night or longer cruises. While it is free, you must obtain a ticket available at Guest Services after you board the ship. There are a limited number of tickets available. I don't believe there's any age requirement (i.e. not children only) but keep in mind that I believe they serve apple juice and cookies, not adult fare, so it's primarily geared towards children - but families without young children are not necessarily excluded if this is what you would like to do. Just stop by Guest Services as soon as you can after boarding.
Enjoy your cruise!