It’s highly unlikely your teenager will want you involved in their Halloween outfit, unless they come to you looking for money for supplies. You can, however, make sure they have a safe and happy night:
Stayr clear of illegal activities
Make sure you teen is responsible enough to stay clear of any illegal activities, including alcohol. Talk about the effects of drinking or using drugs an make sure they know what will happen if they are tempted.
Know where they are going at all times
Make sure you know where your teen is at all times. If they are going to a party, check to see if there are any adults there or if it is supervised at all. Drop and collect or have someone you know and trust do the pick-ups.
Trick-or-treating safety
For younger teens it is highly unlikely they will want you to go trick-or-treating with them. So make sure they carry a mobile phone with them, that is charged and has plenty of credit. Tell them to ring you every 30 minutes or so or to send a text saying where they are. 
Give them boundaries
Give your child trick-or-treating boundaries. Outline how far they can go and which neighbourhoods they should avoid.
Stay together in groups
Make sure they are always in a group and never let them go out alone. Whether they are trick-or-treating or simply enjoying the fireworks, it is important they have at least one buddy with them at all times. 
Give them a curfew
Give them a time to come home at and make sure they are well aware of any consequences if they don’t make in time.
Bonfire safety
Bonfires are extremely dangerous, especially when their are unsociable activities surrounding it. Know which bonfires they are going to and if possible stay with them. For older teens you need to set clear ground rules and guidelines in case things get out of hand.  
Use their common sense
Above all else, make sure you teen uses their common sense. If they are in an area with lots of bonfires or fireworks, tell them to walk away if they feel unsafe.