As the previous poster stated all unused credits will expire at midnight on the day they check out.
However they can "share" them with you at anytime during their stay...i.e., you could dine with them and use 4 of their meal credits instead of just 2 (for your parents) if they wish and I presume if they left early on check out day they could leave you their room key and you could use up anything left in the way of meal or snack credits that day if you signed their name to the bill.