Move Fields permanently from Parent Table to Child Table

Easy.
Go to the table home page on the old report style and click Copy This Table with data

Now you have a duplicate table named new child table and even the Record ID will be preserved.

Now we need a relationship properly populated.

Create a new field which is called for now, [link to parent]

Make it a formula equal the record Id.

Change it back to a numeric field.

Great,  now you have a scalar data entry field that knows the correct value for the Relationship

Make the relationship based in that field.

rename the reference field from 
[link to parent]

to Related Parent (or whatever you parent table is called.

So now just massively hack away and delete all fields that do not belong in the child.