[Gloucs] Lock keyboard/mouse in Gnome without locking screen?

[Andrew Oakley]

I have a two-year old daughter and an Ubuntu laptop (Hardy on a Dell
Inspiron 1520, with Gnome).

How do I lock the keyboard and mouse without locking the screen?

For example, I want to be able to play a video on the laptop, whilst my
daughter to bashes the keys and clicks the mouse/trackpad. Ideally the
keyboard/mouse would only be unlocked once a particularly difficult key
combination or sequence is pressed.

I don't intend to leave my daughter alone with the laptop (result would
probably be Ribena spilt over it), but I would like to be able to sit on
the other side of the room and not have to constantly pull her away from 
it. She is well behaved... for a two-year-old.

-- 
Andrew Oakley