error 2450 with a subform within a subform

[Muzicmn]

ok, am i going about this all wrong?? The problem is i have a db for an elementary school. I created a page to enter student information and used tabs so as not to overburden the front page. I have several tabs and within those tabs several subforms which all use the StudentID field to reference the parent form. The pblm comes in with the Parent form. With several students sharing the same parents, (brothers, sisters) we decided to go ahead and create a separate table for parents and use a table to tie it all together. so i ended up with 3 tables

1 tblStudentinfo (auto# assigned to each student
2 tblParentinfo (auto# assigned to each parent
3 tblStudentParentLink (this table has StudentID (number field) and ParentID (number field)

Next I created a subform for the link table and married it to the main form with the student id, works fine. But when i added a sub form to that subform i wanted the Parentsubform to update the parentid field in the link form. but all i get is error 2450. the code i used works with the parent and link table alone but when added to the main form doesnt

code is:

[Forms]![subfrmSTUDENTPARENTLINK]![PARENTID] = [PARENT_ID]
Forms!subfrmSTUDENTPARENTLINK!PARENTID.Requery

sorry for the long post, just didnt know how else to explain it

thanks

Ricky