FormView is a databound control. Child controls inside an itemtemplate are repeated controls so they don't have global ids like unrepeated controls. You can find them by id but only inside events that fire on the databound control.
I don't use FormView anywhere in mojoPortal so I have no example for you, but it is similar in concept with gridview and other databound controls.
If you look in source code in Web/Admin/ModuleDefinitionSettings.aspx.cs you can see how I find controls inside the Row_Updating event like this:
TextBox txtSortOrder = (TextBox)grid.Rows[e.RowIndex].Cells[1].FindControl("txtSortOrder");
Hope it helps,
Joe