- my dictionary includes several words where 娃娃 seems to refer to a child and not a doll, e.g. 抱娃娃 娃娃兵 娃娃车. It's conceivable that they're all metaphors, I guess.

- I tried once to refer to a doll as 娃, mirroring an actual usage I'd seen, and my then tutor (from Guangzhou) complained that with just one 娃 you're referring to a human.

Having become ashamed of my lazy answer, I just asked a college student in Shanghai; she informs me "you can call a baby 娃娃".