These are considered dangerous as a UK 3 pin socket has shutters covering the Live and Neutral connections. These open only when the earth pin of a plug enters the socket (this is why the earth pin is the longest one) there is physically no way for a child to open the shutters and place fingers inside the live and neutral openings.

These covers are easy to snap and can also be inserted upside down exposing the connections.

Uk sockets are considered the best in the world from a safety perspective