The effect of this was lost on me simply because I couldn't believe a person confusing the taste of baby mice for the taste of cake. I mean, the movement, and crunchy bones, and spurting blood didn't tip the kid off? And as others pointed out, it didn't make sense to have half-mice in the pantry, as that would imply the mice had been halved before he reached in there.
I did like the phrase "they taste almost exactly like peanut butter doesn’t". It rather reminded me of the food replicator in The Hitchhiker's Guide to the Galaxy which, no matter what you asked it for, would produce a substance which was "almost, but not quite, entirely unlike tea."