I think it's important to look at the leaking of gambling into modern youth culture, and the fact that many games include a form of gambling. Online gambling has really led lots of younger audiences to an easier access, which is a huge problem.
That is where parents must enter, their presence and their intervention are the only thing that could stop children from gambling, parents should inspect each game the kid is playing, if there are parts of it that looks like a gambling, or the game itself looks like roulette or images like that, parents should stop their child from playing it.
But anyway, we can't see who are in the gambling site, same as the admins of those gambling site, they can't see the face of the players, so it is difficult to see if a minor is playing in their site or not.