I have two children, and the only thing I worry about is salmonella. Of course, even that isn't a big concern because the kids wash their hands after any touching. It's possible to get salmonella from many other sources as well, so it's always important to wash hands after touching any animal. It's just good hygiene.

I wouldn't hestitate to show a young child a snake, but one that is only 18 months or so probably shouldn't be grabbing at a snake. With parental supervision, she could try the touching it with two fingers. Of course, you have to gauge her reaction to the experience.