Put the sweets out in a box, not a bowl.
Put a note saying you are not participating on your gate or visible on the first bit of the house callers can see, not around the side at the door..
Actually, Mischief Night was always associated with Guy Fawkes night and the mischief wasn't all to do with fireworks. Catholics and Irish immigrants often bore the brunt of the mischief on both Guy Fawkes (and around the 12th of July) in many British cities, esp. Liverpool and Glasgow. Those who compare Halloween unfavourably with Guy Fawkes should probably take a look at the sectarian associations of that particular celebration.
Lots of people know very little about Halloween and Halloween traditions, maybe because it isn't actually a tradition in large sections of Britain. It's convenient to label it all bad because it is assumed to be American, but it originated in Ireland and Scotland, developing from the old feast of Samhain and it has always been a thing in Ireland. There has always been mischief associated with Halloween too, but in America the mischief was largely squeezed out by commercialisation and hordes of cute kiddies thronging the streets with their parents. Traditions like Devil's Night lived on in Detroit, where volunteers still patrol the streets.
Those who object to their local streets being given over to loud and rude elements should perhaps consider decking their children in costumes and taking back their neighbourhoods until late at night.
I am in the US and people who get egged are well known local Halloween Grinches. The eggers tend to be aged 12/13ish.
Last year ds was staying with his cousin and they do Hallowe'en and went trick and treating. My SIL was laughing about all the dressed up teens. Ds and his cousin (aged 12 and 14) were out for 4 HOURS, and came back with 2 huge carrier bags stuffed full of sweets.
I am wondering what you would think of my neighbourhood, where everyone from babes in arms to what are probably 16 year old members of the high school wrestling team dresses up and goes out for the duration of the trick or treating hours, 3pm to 9pm (yes that is six hours) with frequent returns home to empty the pillow cases of candy and go out again. Last year we had snow blowing sideways and my youngest DD aged 13 went out Trick or Treating with her friend and the friend's three little sisters, and again with just the friend when the three little sisters were too frozen to go on, around 8 pm.
Your DS and his cousin are amateurs.