I think skull and crossbones popularly refers to pirates [although why people should want to glorify pirates I do not know] historically they were seen as both a symbol of mortality and of the resurrection.
Routinely allowing children to get terrified by adult horror films I guess things is surely not a good thing, but historically children have been told stories such as Hansel and Gretel and Red Riding Hood, where there are horrible things, being eaten by a wolf, being fattened up to be eaten, but, evil does not triumph in the end; many children are traumatised by Bambi and Dumbo though, one must be very careful. There was a time though when it was thought appropriate to terrify children with the thought of hell and damnation [and the devil] to encourage good behaviour, and nowadays I can imagine that social workers would look askance at anyone who did this. There are some churches [not mainstream] that believe in possession by the devil, which gain is pretty abusive
Keep skulls and away from babies, poor things, but just giving a dummy with a skull on isn't abusive, just in incredibly poor taste