Dependents (I think 16 and above) can use a TM's discount, but should have their own discount card. This includes those in college who are still claimed as dependent by their TM parent.
But a parent of a TM is not a dependent and is not eligible to use their child's TM discount.
As a cashier, it isn't up to you to police discount usage. But if you see any blatant abuse going on, you can let AP know about it.