I think it's extremely hard to explain to young children of the age below 12 why is it bad for them to gamble. First off, you have to be an authority for your kids so that they were trusting your words. Secondly, they may simply don't get what you are talking about.
That depends on how their parents approach to explain to their children. If we can explaining about gambling with carefully and our children can understand about that, they will not trying to gets close with playing. They will not trying to playing gambling even if their friends asks them to playing some traditional gambling to them.
It's all about how their parents can position themselves towards their children and not only strict forbid them not to gamble. But many parents nowadays only forbid their children from gambling without trying to get closer to their children, so this will make their children rebel or try to gambling with their friends in secret.
The thing is that when kids want to gamble, it's not for entertainment for sure. They want to make money from it and money only is what interests them here. It's adults who can(and must) be gambling for entertainment purposes. That's why before you are an adult, you shouldn't gamble.
For the first time, they will curious with gambling and they will search for why people wants to gets fun from gambling. So they will try to playing gambling with their friends in secret and they see that they gets entertainment from gambling. But if they can't control themselves, they will lose their money and they will asks more money from their parents without tells what they will do with the money in truth.
It needs awareness from their parents to explain what is gambling and the risks behind of gambling with the danger that their children can gets from gambling. When their parents can explain everything about gambling, their children will not trying to playing gambling.