What's the deal with the nicknames?

Anna is "Mouse" because she squeaks.  Other than the cry she gave when she was delivered, it's the only sound I've heard her make.

Evelyn is "Kitten" because she makes little mewling sounds that could almost be meows.  That's not the only sound she makes, but "Hannibal Lecter" would invite too much explanation to the preschool crowd.