In that case a kid could have a question, why someone has to play with real money just to have fun. This question sound very reasonable to me. After all, there are lots of other ways to have fun, without risking your money and possibility to end up in debts and with an addiction. What a parent should tell his child about that? I get your point about the necessity of drawing a line between gambling and earning. But my point is what is the argument to support precisely gambling as a way of having fun?
I have a pretty simple explanation for a kid in this case: "adults can use real money to gamble to have fun, when you grow up and have your own money, you can do with them whatever you want." And additional argument "adults can ride real cars, but because you are still young, you can ride only bicycle or car with pedals". But like you have said, there is plenty of ways to have fun, that is why kids wont pay much attention to gambling, as he will get fun from more simple things, the things that he understands.
My idea from the beginning was not to teach gambling as this is a way to have fun, but not to hide it on purpose, and if a kid ask, give explanation, give examples. I dont support teaching on purpose, but if kid noticed you gamble, just give an explanation of what you are doing. Instead of quickly closing browser or cleaning table from chips, cards or whatever is used to gamble.