Okay, so I just tested this on my GOS phone by rebooting the phone.
If the tablet has rebooted, you will have to log into the owner profile once, to fully unlock the file system. This is a security feature to protect you against theft.
Assuming that you, the parent, have logged into the owner profile and then locked the screen, your child is able to log into his/her profile from the lock screen.
1) click the power button or double-tap the screen, whichever brings up the little lock icon on the screen
2) Have them touch the screen near the top and then swipe all the way to the bottom.
3) At the bottom-right of the screen you will see three icons. The one on the far right is power, the one next to it is the cog-wheel of settings, the third from the right is the user icon. Touch the user icon, and a "select user" dialog box will appear. Touch the icon for the desired profile.
Hope this helps!