I don't think they are allowed to accept tips....we gave Matthew (who was fabulous!) one of our Cast Member Appreciation Cards I had made up. I did business cards expressing our sincere appreciation and thanks with cute Disney clip art and sprayed them with spray on glitter (my personal pixie dust) and laminated them....we gave them to various Castmembers who were especially nice!
My DH tried to give our tourguide (David-Logan) a tip after the Family Magic Tour last October. He refused & said he could not accept a tip. he was an absolutely wonderful guide & did great with the kids...I guess they are told by their superiors that tipping is not allowed.
Probably a great thanks if you really like your guide is to stop at City Hall and fill out [I think it's called] a CM appreciation card. I've never thought to so this before, but I definitely will my next trip.