It largely depends on what they are addicted to. Get them into therapy. There is no substitute for this. There are many good out patient programs as well as inpatient clinics. Good luck.
Children under 18 can be placed in drug treatment programs by their parents and there are drug programs designed for teens. Find out what your options are in this area.
You need to decide if you want to be a friend to your child or if you want to get them cured. If you want to be a friend, then give them a safe place. If you want to cure them then buckle in and ask for help.