If you’d like to remove a child/list from your account, just head to your membership account page, then click “Manage Book List” next to the child you’d like to remove.
You can then scroll all the way down to the bottom of that page, and find the button labelled “Delete List”. If you delete this list, it will permanently remove the reading record, certificates, and progress.
It won’t be fully removed until you’ve also returned all the books currently on loan to that child.