Translated by google:
>>Removing the family group after requesting the removal of a minor
I requested the removal of a minor from the family group; I am the administrator and want to leave the family group. The minor no longer appears on the list, but the system won't let me delete the family group. How can I fix this?<<
You will have to move the child first to another family group, before you can turn off Family Sharing as the organiser.
If the account is not listed anymore, you can get the Apple Account deleted by Apple, but will have to wait until this happens, before you can turn off Family sharing.
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Children under 13* can't be removed from a Family Sharing group without being transferred to another family group, or having their Apple Account deleted. To delete their Apple Account, a child can sign in to their account on Apple's Data and Privacy page and select “Request to delete your account." The family organizer will receive a request to approve the account deletion via email. When the family organizer approves the request, they'll go through the process of deleting the child account.
It takes time to process the deletion of the child account. The family organizer will not be able to disband a Family Sharing group or delete their own Apple Account while this request is processing.
Learn how to delete an Apple Account
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copied from: How to leave or remove a member from a Family Sharing group - Apple Support
Cómo abandonar un grupo de En Familia o eliminar a un miembro de él - Soporte técnico de Apple (ES)
How to transfer a child to another Family Sharing group - Apple Support
Cómo transferir un perfil infantil a otro grupo de En familia - Soporte técnico de Apple (ES)