With Royal Caribbean the 3rd /4th passenger in same cabin regardless of age pay a 3rd /4th passenger reduced rate. The are no child reductions but usually reasonable reductions for 3rd/4th in same cabin. If you go through the booking procedure on rci website you can bring up the prices without taking it far enough to final booking stage. Hope this makes sense.
Call 0844 493 4040 and speak to RCI or call the number at the top of this page...
Also go to the RCI website & go through the booking procedure. That will give you an instant price, itemised including the 3rd person. Take it right through to the point of 'continue reservation' & then just close the website, it's commonly known as a 'dummy booking'.