What copyright laws apply to custom cake orders? I get requests for replicas, do I need permission?

I have had requests for characters (hello kitty, winnie the pooh) and objects (beer cans, ipods, cars, etc.) What sort of copyright laws are there regarding this? I am starting a small business and currently make cakes on a freelance basis. Also, I do not make the replicas and then sell them, they are requests per customers for their custom cake. Any help on this is greatly appreciated!