On the phone:

If you no longer want to use your child’s current HUAWEI ID account, you may delete it by logging in to the child account or your device and then going to your Settings > HUAWEI ID > Privacy center or Security centre or Settings (depending on your phone model) > Delete account.

After that, the parent HUAWEI ID, whose the child account is linked to, will receive a verification SMS with a link in its security phone/email address. To complete this procedure, the parent should CLICK on the link and then APPROVE the red marked Account deletion request on the PRIVACY CENTER. If you delete the HUAWEI ID child account, the personal data related to that child account and the Huawei mobile services your child has used will be erased.

On the website:

On the computer browser, log in id.cloud.huawei.com > Child accounts > Delete child account,Select the account you want to delete, and follow the prompts.