From @OP's story, we see children are used to saying things like Let's bet easily. We don't know where they got those words from, but usually, they come from one or several children who are used to seeing and hearing those words from the adults around them. They then look for the meaning and use it on their friends. And look, they have known about gambling since they were little because their friends already knew about it and invited them to play. Especially if they have a smartphone at an early age, they might see many advertisements they shouldn't watch.
Apart from that, lottery games are installed in shops to attract children to play them. As parents, we often don't realize that we are allowing our children to play with luck, which is gambling. Gambling can easily enter our brains, make us remember it well, and play several gambling games that we didn't even realize we had when we were little.
I think it is also part of the popular culture which both children and adults adsorb from social media, Television and even folk stories.
If you try to recall, you could easily remember yourself as a child watching cartoons and there was some specific episode where the protagonists bet against each other, for some silly reason.
There is where children learn about gambling from, right in front of the eye of their parents.
I can recall watching several examples of betting and gambling on shows like Kennan & Kel, Spongebob, KND, etc. Before you notice, your kids already discovered what a bet is and how they are supposed to work. It only takes two kids in the school to know and they will start to bet on frivolous things, which are at first harmless.