What proof exists that Japanese soldiers bayonetted babies at Nanking?
I don’t specifically know about the bayonetting of babies, but there is photographic evidence available of other atrocities. What I remember from the book I saw was photos taken by the Japanese soldiers of other Japanese soldiers with their victims. I remember rape and mutilation in the images I saw. There are a number of books on it. I remember the book I saw had an eye witness account of, I believe, a Western priest. Wikipedia has this to say.
According to the Nanking Massacre page at Wikipedia, the evidence consists of “Witnesses recall[ing] Japanese soldiers throwing babies into the air and catching them with their bayonets.” There’s a bibliography in the article that might help you find the direct corroborating witness statements. According to reviews of John Rabe’s diaries at Amazon.com, he was one of the witnesses who specifically mentioned bayonetting of pregnant women.