XZ1 Dual-Sim blocked

[Kropik]

I gave my son my old XZ1.
In Google Family i've created a new account for him, and all was good.
He put up the pattern option to unlock the screen. 
Now the patter he claims is the correct one doesn't work. Any attempt results in a 60 second timeout. The same applies to all other pattern gestures.

I was only able to connect the phone to my wi-fi.

I tried the following to unlock the phone:

1) Factory Reset
Key combination only gives me 3 short vibrations, and the screen stays dark

2) Flash Tool
Doesn't do anything, it requires the device to be unlocked

3) Sony Xperia Companion

I can't get anything done. App said that the device needs t be unlocked and through "Help" i learned that i need to run a Software Repair. But even that tells me that it's required to unlock the device.

4) Google Family
I went into the "Reset device & delete data" section, and asked it to reset the device. The device disappeared from the list of devices, but  nothing happened on the device itself

5) Find my device
I tried this feature, but it may not be turned on. The device was not located.

6) Bootlader

I tried this, but i can put in the code " *#*#7378423#*#*" only in the emergency call feature. It doesn't do anything.

7) Hold "Power Off"
I also found that i should try to bring up the Power off option, and then hold. it. Unfortunately, that only brings up the security pattern again.

Can you pls advice what else i should try, or maybe i'm doing something wrong?
Can you pls advice what else i could try to  

[Strampke]

When you hand over a phone to someone else you should delete your account on the device before doing so.

The device is 'attached' to your last account to prevent access to third party.

Maybe you can find some additional information here.

[Kropik]

I tried Find Device under my Parent account, i tried under my son's account. Find Device doesn't reach the device.
I couldn't find any help in the link that you provided.
Can you point me to a specific solution i should try, or haven't tried yet?

Is it possible to do this directly from his (or my) google account when i sign-in on my PC?