The admin user on an account can reset the password for a child user. In order to do this follow these steps:
Log into your admin account at https://account.mirageid.com
Click on the "Users" tab, and then select the user you want to reset from the list:
Then click on the reset password button for that user:
You will be asked to confirm, and then be prompted for *your* admin login:
Enter your admin password, and the user will receive a password reset email. Note that that email will contain a temporary 1-time user password, and they will need to reset their password again after they log in.