There are two passwords associated with the KoalaSafe. The app password which only adults should know used to login to the KoalaSafe app. This password can be reset if you forget it.
The second password is the WiFi password. This password is given to your children and they can use it to connect their devices to the Koala WiFi. This password can't be reset, but it can be changed in Settings --> WiFi Settings.
It sounds like you may be using the app password for the WiFi password?
I would suggest changing the WiFi password to something like 12345678 then trying to connect a new device