I know its been a couple months since you submitted this, but I had a similar situation today and thought I'd add my idea that worked for us.
You could consider creating a formula - checkbox field and name it "Product Request Validation" or something similar and have the field sit in the background of this form. Then create a form rule that says when the record is saved, if the box is checked, then abort the save and display a message that says "No Products Added" or something.
Next create a summary field (also sitting in the background) from the child table where product requests are added. The summary field would total the number of records with the related unique identifier to this specific product request form. Then the formula - checkbox validator on this form will be unchecked for any time the summary field number is greater than 0.
In theory, this would prevent your users from saving this form if empty, but allow it if product requests are entered.
