According to US Baby experts:

If your baby inserts fingers or other objects into the socket, it may result in electric shock or short circuit. Using insulating socket jacket for the socket holes, your baby can no longer insert fingers into the socket and therefore the risk of electric shock is avoided. US Baby insulating socket jacket can be inserted into the socket in just a few seconds, and it is difficult for your baby to take off but quite easy for adults. It is suitable for general sockets, convenient and practical.