What is the database schema for groups and parent groups?
Groups are stored in civicrm_group.  Parent/child relationships are stored in civicrm_group_nesting.  However, based on the error you've posted, that's not the issue.  The issue is that the group was deleted, but there are still contacts who are a member of the group (which is stored in civicrm_group_contact).

I'm not sure how you did that - as the error reports, there should be a "delete cascade" when you delete a group to remove the civicrm_group_contact records.  To fix this, make sure you have a backup, then:


Temporarily disable foreign key constraints.
If your group id is 54, run DELETE from civicrm_group_contact WHERE group_id = 54.
Re-enable your foreign key constraints.


Hopefully that should clear this error!