I have friends who have been. They enjoyed it during the day, but found the night times a bit of a waste because the kids needed to sleep and someone then had to stay in the cabin with them and missed out the evening stuff.
On just cruising in general, my mum is a cruiseaholic and her advice is to go with the best cruise company you can afford. P&O are really only 3 star (in her opinion) and companies like Royal Carribean and Holland America are much higher quality (food, accommodation, entertainment). I hear Carnival are family friendly but not sure about their quality.
I went a few years ago with family and friends of my parents. One of the couple had 4 boys under 6, one who had special needs. They loved it! Mum and Dad got to spend some quality time together and the kids all had a great time in the kids club. They raved about how wonderful it was for quite some time.
Bookmarks