Ah, two cultures seperated by a common language. This could be difficult to explain without pics but here goes. A safety pin is just a bent length of metal wire with a loop at one end to act as a spring and a catch at the other with a fixed end and a sharpened end which can be released. Same both sides of the Atlantic. What I was refering to as a nappy pin tends to be larger and has a white (at least any I've seen are white) plastic sleve which can be moved over the catch (a sort of press fit ) to stop the catch being released. The pin cannot be opened without moving the white catch out of the way. Used for nappies so that when the baby is crawling about on the floor the catch will not disengage and stick the baby with the pin.

Hope that made sense without pics. I have seen the term blanket pin used on a forum (not sure if it was this one) with someone from the US saying they had safety pins and a couple of blanket pins in their PSK. Is this the same thing?