That is between you and your dear wife. A portion of the tips go to food service employees, will your children eat on board? Will your room be more or less tidy with four people occupying the cabin instead of two. Will you use more beds, towels?
My advise - only adjust the tips if you receive inferior or superior service, but they are your tips and you can do with them as you like.
All I ask is a tall ship and a star to steer her by; John Masefield
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, 2014, Caribbean 2013, Coral 2014, Regal 2014, 2016, Royal 2017
Star Clippers: Royal Clipper 2015