When our son was young (pre-teens) we would always tip key people in advance and let them know that we really appreciated their efforts at keeping an eye on our son. It worked great.
My wife's Mom became ill with Alzheimer’s and we continued to cruise with her long after most people would have. The day that we boarded the ship I would find Elizabeth's cabin steward, explain that Elizabeth needed special attention and tip them $100 on the spot. Elizabeth became famous instantly on the ship and everyone looked out for her.
Tipping can certainly improve your experience on a cruise, in my opinion.