From the image it looks like they're in resin. I agree that people should be careful with jewelry around small children, but it doesn't look like these are the type that will open and spill out their contents, regardless of it the contents are poisonous or not. I feel like there would be bigger concerns if the baby bit a chunk of resin off. Maybe I'm mistaken though since I'm just going off the pic provided.

and that's a greater hazard than the materials inside the locket, so kind of a moot point