I think its a little different in regards to tipping on a cruise compared to tipping at a restaurant. When you book your cruise, you get all the information stating what amounts are approperate tips for the various staff members that will be serving you, its in print in every cruiselines brochure, and should be considered a cost of cruising. These guidelines are pretty specific and if tipping on a cruise doesn't fit in to anyone's budget, then the cruise isn't the vacation for them.
I don't think anyone is calling you cheap, but when you are on a cruise you have a lot of service people making sure everything is right and they go out of their way to make sure things are what they are supposed to be. They truly deserve the tips they get, thats how they make their living, plus they work long hard hours and hardly get any time off.
When we cruised with our kids, regardless of age, we always tipped the regular amount for them, the staff work just as hard or even harder to make sure the trip is fun for them.