We have traveled twice with grandchildren. The first time, we downloaded the Carnival form letter and had both parents give notarized signatures on the forms that we could take them with us. No one even asked to see them.
Last month, my daughter traveled with us and she had her children. She has joint custody and had the father sign (notarized) letters for each of the children allowing her to take them on the cruise. At the check in desk, every single letter was scrutinized by the clerk.
I would say "better safe than sorry" and have the proper docs with me when I checked in.
Been there - done that:
2005 Holiday, Sensation, Conquest,
2006 Conquest, Celebration, Holiday,
2007 Freedom Grand Med, Holiday
2008 Fantasy & Sensation,
2009 Fantasy, Holiday & Dream Grand Med
2010 Fantasy and B2B Elation
2011 Monarch of the Seas
2012 Booked - Breeze from Barcelona 12 days