This is perhaps the wrong lesson.
One of my favorites is Zombicide: https://www.zombicide.com/en/
Although I'm not sure it's suited for kids, when I start playing I always get hooked and then it's sleep time haha.
Forbidden Island is a classic, probably best for slightly older kids
https://boardgamegeek.com/boardgame/65244/forbidden-island
Outfoxed is easier for younger kids: https://boardgamegeek.com/boardgame/172931/outfoxed
Powergrid and Agricola are two examples.
Settlers of Catan would be a good example with positive sum trading.
Players cooperate to get a bird, a chipmunk and a mouse safely to a tree while a cat named Max is chasing them. Players roll dice that decide how much the cat or other animals move, and they decide what order to move each, or whether to distract the cat with milk or catnip.
All players play together toward improving the farm.
Monopoly does not count as positive sum IMO because when other players get rich, you end up paying more rent. Cooperative games where multiple players are good vs a few bad ones may be positive sum.