I don't have kids but have cruised quite a bit, recently went on Royal Caribbeans new ship Freedom of the Seas, check it out, there is a fantastic amount for kids to do, from ice skating to rock climbing to body surfing!!! True, they also had several kids clubs for the different age groups. Other cruise lines have kids stuff but these seemed to be a cut above in compariso.
Most of the Royal Caribbean Ships have very good facilities for children, Ventura is being launched for 2008 and this is a very family orientate.
P&O's Oriana and Aurora have first class kid's facilities, including their own swimming pool and deck area. We took our grandaughters, ages 7 and 9, last July/August on Oriana and they had a great time. Wonderful activities and staff. Have also seen good facilities on Royal Caribbean and Celebrity but have no knowledge of how good the staff are. Having cruised 20 times with various lines I think the above cruise lines are the best for children.