I believe that RCI asks for full tips from any passenger, including infants. It's not just the dirty disposables (where would YOU put them?), but setting up the Pack & Play (either yours or theirs), bringing juice, etc., to your cabin, supplying a high chair in food venues as well as food, and basically making sure that your cabin is clean and you are satisfied. BTW - my understanding is that there are only a couple of RCI ships that will allow NTT babies in swimmies in pools, , and that is only in a very small designated pool. Can't go in any of the main pools. And - most RCI ships don't have drop-down diaper changing tables in cabins or public restrooms. Check RCI to see if you're cruising on on of the newer ships that MAY have these features. Pay the tips.
NTT - Non Toilet Trained