They will have their own cards. They need them to get on and off the ship.
I'm not sure about funding their cards, we always put ours against a credit card. That way we avoid the long lines at the pursers desk at the end of the cruise.
A Bad Day At Sea [with power] Always Beats A Good Day At Work
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, 2014, Caribbean 2013, Coral 2014