They are intrigued by the heat and different smells of a human's breath. My cat will do this and try stick his paw in my mouth out of curiosity. They are curious and explorative animals. For the record, cats do not PURPOSEFULLY kill babies. Their smells, body warmth and smallness simply make them interesting for cats to smell, explore, cuddle, etc... Cats like to cuddle and sleep on chests of adults too, but this isn't looked upon as sinister as lying on top of a newborn or young infant.