you have too many tables in this.
you ONLY need the master table and child. (not the product table)
the product table is only needed to pick options. Its not part of the master/child relationship.
Order -> orderLine
Now for frmMainDay, your master table is: DAY, w key [dayID]
so the sub form ONLY needs a table with fields: [dayID], and [something] (whatever youre tracking)