I'll add when I worked in quickservice food and beverage, they told us we had to decline 3 times and then we were allowed to take it. This usually had to do with tips, but I guess if you were offered something else you could have accepted it. Never happened to me, but characters are quite different as well. I was unaware we couldn't ask for food doing a dinner meet and greet (was totally joking around when I pointed at some shrimp and pasta at the buffet) and they gave me a to go box anyways, but I'm about 90% sure I could have gotten in trouble for that. (also note, this was when I worked at Universal, not Disney, so different company and different rules)
I'd say take it and if anything give it to their greeter or Peter Pan and see what happens, he'll probably be happy to get it.
The cards you are talking about are called "great service fanatic card"..i received 5 or 6 during my CRP program at EPCOT and it's really an amazing way to receive recognition from the guest for giving it everything that you have. twice even my managers filled out those cards for me because they witnessed me making a guest very happy :)
When I visited WDW in Sept 2012 I made it my goal to fill out at least one service fanatic card every day - recognition for their services is something that CMs unfortunately get too rarely, so this is what I would recommend you doing if you want to tell someone that he or she did a great job (and it makes the CM eligible for some kind of "lottery" where they can win - I think - cruises or such. Chances of winning are linked to the # of cards that a CM has received)