Admin can't select user to reset password This relates to a 2011 Macbook Pro running El Capitan. 

I'm set up as the admin and my child has a standard account with parental controls enabled. My child forgot their password and I'm trying to reset it in System Preferences. I unlock the padlock with the admin password, but my child's account is still grayed out and I can't select it to reset the password. 

This also happened about a month ago but one day suddenly I could select their user account and I successfully reset the password. I don't know why I suddenly could select it, and I don't know why I now suddenly can't select it. I tried locking and unlocking the padlock but it doesn't make a difference.

I know how else I can reset the password, but what I'm asking here is:

What circumstances could explain why I can't select the user account after unlocking the padlock with my admin password?
The system won’t let you reset the password if it detects that account is currently logged in.

Obvious cause is fast user switching and the other user is logged in.
Less obvious cause, things are messed up.

In case 2, try reboot and safe mode boot or disabling automatic log in to be sure the other account(s) are clear before going deeper.