I respectfully disagree, Spritfilled. We've taken our son on cruises every year for 12 years (he's 13 now), and kids are extra work for the crew. For example, our cabin steward would have another bed to service, often one that was stowed away in the wall during the day and opened at night. The dining room crew still served his meals, modifying their schedules as the timing of kids' meals is not necessarily the same as that of the adults. Plus, the child is occupying a seat in the dining room that might otherwise go to an adult. In fact, we usually tipped more for our son.
Some lines (NCL is one) suggest tipping 1/2 of the regular rate for kids under age 12.