As a parent, you can help your child log in from your parent app. First check if the login credentials for the child account is correct. To do that, go to:
Settings
My children
Select child
General settings
Check that the email address or phone number is correct. If not, update the credentials.
Now your child can log in to this account, using the phone number or email address that you provided. Make sure not to add the same email address or phone number that you as a parent have registered to your user account in Gimi! You can only use credentials that are not already used by a user account in Gimi.