Lets see if I understand what you want:

The existing: (as of yesterday)
Source Table -

Parent | Child
-------------
1234 | 13, 27, 189, 210





Today, a user edits the child field for Parent = 1234 so that it is
Parent | Child
-------------
1234 | 13, 27, 189, 210, 300

After running the query for Parent 1234
the result is

Parent | Child
-------------
1234 | 13
1234 | 27
1234 | 189
1234 | 210
1234 | 13
1234 | 27
1234 | 189
1234 | 210
1234 | 300


So my question is "Do you want the duplicate child values or should only the 300 be added?




Edit: if you already have a dB, maybe you would post it (minus sensitive data and only a few records as examples)??