The earth-pin opening system is what is used on most BS1363 sockets and has been a requirement since the standard was introduced in the 1940s.
A similar system is also used on BS546 (the old round-pin sockets).
It has proven to be quite safe and it's not very easy to override, certainly not for a toddler anyway.
However, I agree the MK approach which uses the two pins at the bottom is better. Some of their newest designs actually require all 3 pins to be inserted to open the socket.
The major thing in the BS system is that *all* sockets are shuttered, where as it's a relatively new or optional thing in other countries.
