But you have to go our of your way to run this way. And if you were to run this way there is still nothing to prevent a child from logging on to some other server (there are thousands).
With some mods, it can also teach basic programming skills and simple electric circuits.
It can be modded into a pvp shooter. Not the worst thing in the world, but it sort of kills the educational value of it. It is not a safe, walled environment because, you know, other players are coming to kill you. Moreover, even in other forms of minecraft other players can come and destroy your stuff. They can also yell at you and call you names and otherwise annoy you via the chat window.
Didn't you RTFA? They were built by Cisco.
RTFA?... but isn't this slashdot?