Tradition? Not really. It’s just a thing that we in North America have been doing for the last 40 years or so.
It’s all good fun, provided you are judicious in how hard you hit. For example, I wouldn’t wind up and give some 6 year old girl a full-on whap, but I wouldn’t be overly shocked if her 4 year old brother hit me with everything he had.
The etiquette up here also insists that only the shoulder or upper thigh is a fair target, and “PUNCHBUGGIE no returns” ensures that only ONE person gives and gets the strike.
( I can’t believe I’m posting the PUNCHBUGGIE rules! LOL )
When I see her Thanksgiving, I'll tell her this is an evidently long-standing custom, and whap her again.
All my kids play punchbuggie.
The object was to be the first one to see a Bug, call out the color, then slug someone.
If you called the color wrong, you were slugged back.
You could only be slugged once for each Bug.
If you slugged the driver, all rules went out the window.