syscat.references
TABSCHEMA/TABNAME shows children
REFTABSCHEMA/REFTABNAME shows parents

It is in there somewhere. Try looking at the constraints.

This all depends on depends on the declared foreign key constraints. If
you have defined the relationships, then this is possible. Serge
pointed out that these can be found in the SYSCAT.REFERENCES table. In
order to traverse the tree structure, a recursive query can be used.
Assuming you have the same schema name for all of the tables,  the
following query should give you a list of parent/child relationships
including how many levels deep the relationship between the top level
table and any subservient table:

WITH parentchild
       (
               parentname,
               childname,
               depth
       ) AS
       (SELECT reftabname,
               tabname,
               1
       FROM    syscat.references
       WHERE   reftabname= 'DEPARTMENT'
       UNION ALL
       SELECT  child.reftabname,
               child.tabname,
               depth+1
       FROM    parentchild parent,
               syscat.references child
       WHERE   parent.childname = child.reftabname
       )
SELECT  DISTINCT parentname,
       childname,
       depth
FROM    parentchild
ORDER BY depth

Hope this helps,
Evan