Being Italian, Costa carries huge numbers of children (800 on my cruise), who are catered for in the Squok Club on Deck 10. Being Italian, most of the passengers go nowhere near it and keep their kids with them all day, swarming over the pools and running around the ship.
Although babysitting is offered, late at night Deck 5 was packed with exhausted toddlers crashed out in buggies or slumped on tables.
In the club (Squok is its mascot, half dolphin, half shark), kids are divided into three groups: 3 - 6 year olds, 7 - 11 year olds and 12 - 16. There's always an English-speaking counsellor on hand. Activities are pretty standard: treasure hunts, games, books, movies and computers. Rather sweetly, there is a children's choir on the Christmas cruise.
Interestingly, there are two parallel policies onboard regarding alcoholic beverages. European kids can drink at age 18 while Americans have to wait till they are 21!