Login Ubuntu with no password for children like in windows

If you wish to use Ubuntu 12.04 and let a user open an session with a simple button click  (without being prompted for a password). Just type the following command in a terminal window :

sudo passwd -d myuser (where myuser is the user you want to grant this right) 

The command clear the password in /etc/shadow file for the specified user. You can then open a gnome session without any password.
Simple isn't it ?