Easy user switching program for Windows 7?

Why not just use the built-in user switching? If you want to create a shortcut to switch users, you can make a shortcut on the desktop to tsdiscon. Running this shortcut will disconnect the current user session, then they can click on their username on the login screen to access their account.

I use Direct User Switching Task to switch between domain accounts. You can use either the system tray icon or a keyboard shortcut to switch between accounts in a similar way to switching between apps with ALT+TAB.

This may be a bit too fiddly for five year olds, but I thought I would mention it for the benefit of other readers. It would also be quite trivial to automate its use using AutoIt.

It's not free, but worth getting if you switch between accounts a lot.