Advice about a parent-child relation design

I have 2 tables: parent and child. One of the fields, which should
supposedly be part of the parent table, might occassionaly be different for
one of the child table records.

ParentTable
* id
* date
* customer <-----
* reference
* other fields...

ChildTable
* id-parent
* document type
* customer <-----

It is possible that the customer field is not the same for all of the child
records. This is a very specific (and strange) situation that might
eventually appear in the information system I am developing. I suppose that
such a field must be part of the child table even if this case does not
appear frequently.

I am writing to ask for your feedback and see if there is another possible
approach.

With respect,
Jorge Maldonado