Refering to sub form controls is dicey. Is there a reason to do data entry in a sub form? It sounds like it is a data sheet sub form, which makes it even more difficult.
The problem with referring to sub form controls is naming them properly, then taking the Control.OldValue and replacing the new Value without replacing it in every other control on the sub form.
I would consider a design change. What does the sub form look like?
mmcdonal
Look it up at:
http://wrox.books24x7.com