How to reset a non admin user password in Lollipop? My son forgot his password in my Nexus Tablet. He does not remember the pattern to draw. Is it possible to reset it, or I must delete his account and recreate it?

It used to be possible. Android used to display an option after a number of failures, but it does not work anymore in Lollipop. 

I have the password if the admin user. Lollipop does not display the forgot pattern link anymore. If I downgrade, I'll loose all my data.
If the device is rooted, resetting the PIN/Password/Pattern involves the same dirty tricks that you would use if Google credentials doesn't work and you don't want to do factory reset.


Install a file manager with root explorer abilities such as ES File Explorer or MiXplorer.
When you're logged in as the primary user, use the file manager to go to /data/system/users/10/ and rename:


password.key &rarr; password_old.key // to reset PIN/Password
gesture.key &rarr; gesture_old.key // to reset Pattern


10 is not absolute here. You'd have to identify which userId belongs to the account whose credentials you want to reset. 
Changes would take effect immediately but you may consider restarting the system.
Switch to that secondary user. You can now use any Pattern/PIN/Password and it would unlock the screen. 
You can also set a new lock-screen security from scratch.