I brought the Mudita Kompakt for myself and a second one for my ten year old daughter. For holidays at first and later as her real phone if she shows enough maturity. The Mudita is a great phone for a child because of the minimal nature.
I was showing her the phone and explaining we will use it on our next holiday and well she managed in the space of five minutes to put it into some phone diagnostic mode. with Chinese characters. I managed to hold the power button down to turn it off and it went back to normal operation, phew 
But I suppose is there anyway to disable this diagnostic mode from occurring, or how did she enable it in the first place so we can avoid it. If you have children it is amazing how they can tweak stuff you weren’t even aware was possible. Little gremlins 
She was only on the lock screen of the phone with the unlock code being her date of birth (dmyyyy) she won’t forget that number. Google search suggests some phones have a code you can enter at the keypad. I was wondering if that was how she did it.