How and what do I do to get the tenants downstairs to move out? Any tips or ways to rid someone? ?

The only thing your parents can do is build a "no smoking" clause into their lease. They have to wait til they renew their lease, or put a legal addendum to the lease. They would have to provide guidelines, such as "no smoking within 50 feet of the property including porches, balconies, and parking areas"...If the tenants break this clause, your parents would have the legal right to evict them. Or, maybe it would be just enough of a pain in the for them that they'd see the new rule and move out because of it. Anything else that you or your parents do to try to push out the tenants is illegal. Sorry for your situation...I used to be a smoker myself, and now I can't stand the smell and I absolutely freak out when someone smokes around my baby. Hope this helps. Good luck.