Because that laugh doesn't make his intentions obvious at all-oh well. He was supposed to be playing innocent, helpless, and naive little boy so he chose not to react and just looked at the man's hand. If he could get Fish far enough away from the room then someone could slip in and save the Bunny. It might be pushing it but--

"Sure! But, can Plot Bunny-san come too? I bet he'd like a carrot!" Conan smiles happily. A small seven year old would want to hold the rabbit, after all.