When pairing your child to your subscription, ensure that you are using the correct code.
Don't use the 8 digit voucher code. Instead, have your child scan the QR code in your ChatLicense app, or fill in the pairing code manually.
Have you just downloaded the app for the first time and are you met with an error when immediately trying to pair your child with the 6 digit pairing code? Then you are probably in the log-in page for children who have been paired with a parent before. Your child needs to start a new adventure and complete the chat with Moira (fictional character). Along the map of Chat City, in the bottom right, you will find the button with which you can link your child for the first time.
Find a more detailed explanation of how to link your child in the app here.