Put your foot down and tell her that the snake killing the baby is all in her head. How many stories have you even heard of that happening? Zero. The odds of the snake getting out, the baby not being monitored, the snake GETTING to the baby and doing any harm is about 1 in 1,000,000 chance. How big is the snake right now? If it's 6 months old, it'll take a long time before any harm could be done.