Not sure if people have seen the latest edition of the Primary CLEAPSS magazine. In it there is an article about the safety of socket covers in school. Their advice is that you do not need socket covers and that they are actually inherently unsafe.
All plug sockets have shutter mechanisms that close the pins when not in use, if socket covers are used this shutter mechanism is opened and may not function as intended, especially if the earth pin is snapped off.
For further guidance I recommend you contact CLEAPSS