Cruises typically charge full fare, or at least standard 3rd/4th person fare, for children (there are occasional "kids sale free" promotions on some lines). You could probably find a bargain on an inside room, but the way we cruise- daycare is cheaper at $250 a week.
The daycare aspect (kids club)also is often not full day. There are blocks of time here and there. Some lines will take care of feeding the kids, others require pick up during meal times, others let you pay extra to have the kid there in meal time. There are generally night hours which are an extra fee, or you need to hire in room "babysitting". (And on some lines while kids club has vetted staff, babysitting is any member of the staff, like say a housekeeper picking up extra hours.)
Children who are not potty trained are often excluded from the clubs, or else you are paged to come and change the diaper when necessary. Kids in diapers generally cannot go in pools on cruises as they are salt water, not chlorine, so a swim diaper is not sufficient (I think Disney is an exception.)
Depending on the line, programming during these days varies- but a lot of the program is "free time" where kids just play video games.
I will say though- we LOVE cruises.
I'm not sure about cruising with kids, seems really stressful to me; and being separated on the boat terrifies me in the case of an emergency.
