As an existing Heja user you may need to join another team and and connect to your child in that team, follow the below instructions to get setup:
Start in the Team tab and long hold the Team icon, or click on your team name to bring up your team list
Choose the Join team button at the bottom of the list
Enter the team code & press continue
Choose join as a parent to a player
You will then have the options to connect to a child, add a new child, or bring an existing child with you as follows:
-> 
Here you can see that your child is not yet in the team. Choose "my child isn't listed" to add your child, or to add a new child.
Here your child is already in the team under "best match." Connect to your child by ticking the checkbox.
If you do not want to connect to your existing children, you can always:
Connect to another child: Choose another child from the list if you haven't connected yet ticking the checkbox beside their name.
Create a new child: Select My child isn't listed > Add a new child.