Storing history of a hierarchy changes in database

Question

I have a class that uses a hierarchy structure:

    Class
    {
       string Id;
       string ParentId;
       //... other properties around 30
       Date ModifiedDate;
       Date CreationDate;
    }

We have some kind of audit trail but is something like:

    AuditTrail
    {
       string ClassId;
       Field FieldId; // with an enum for each property
       string OldValue;
       string NewValue;
       Date Date;
    }

Now the customer wants to see the hierarchy structure at some point in time. (a Class can have only one parent but it can have multiple children)

Example

A is parent for B and B is parent for C

on a point in time

D is the parent of C

For a date before the change the hierarchy should be A and B and C

For a date after the change the hierarchy should be A and B

Using the audit trail in the actual form would be a nightmare to determine the entire hierarchy at some point in time, so I though of storing the entire structure in an xml with a validity period. (The reason is that it needs only 4-5 properties to know at some point in time).

Are there other better solutions for storing the hierarchy changes