Restrict User Access to Page in Relational Database

I have a relational database with two tables linked on a common ID field. The logged in user can access all of their entries in the child table with simple SQL queries and then select from a list of matching records which of their records in the child table they wish to edit (i.e., the ParentTable['ID'] == ChildTable['ID']). The record is then displayed using a $_GET passed through the URL as a 'recordID' parameter. However, when the user is logged in and they access a record that matches the query, they can then type another 'RecordID' number into the URL and get access to any record in the child table whether or not they are the 'owner' of the record.

I have tried putting an equivalence statement in the user authorization code to restrict unauthorized access to child records from logged in users since the ParentTable['ID'] == ChildTable['ID'] only when logged in user accesses the records they previously created. (In other words, when a logged in user types a different 'RecordID' into the URL, the ParentTable['ID'] and ChildTable['ID] are not equivalent.) The code I entered into the user authentication generated by DW is as follows:

if ((isset($HTTP_SESSION_VARS["MM_Username"]) && ($row_ParentTable['ID'] == $row_ChildTable['ID']))) {
...

Access is still not restricted even though testing shows that the ParentTable['ID'] and ChildTable['ID'] are not equivalent

Any ideas on how to restrict access to the 'unowned' child records? I am sure it is relatively simple but I am having difficulty getting through this obstacle.

Thanks