Gambling is prohibited by law, as is selling alcohol to teenagers, but this does not mean that teenagers do not drink alcohol. The same applies to casinos, not all casinos require verification, but a teenager needs to think about where to get money, and how to send it to the casino to start gambling. If the casino allows a teenager to play, they will also be responsible for this, so this is not only a problem for parents who have not explained to their children all the responsibility for this.
At least, if parents explain and educate about responsibility to their teenager, they will not trying to playing gambling because the risks behind gambling. Teenager will be careful when they socialize with their friends or will not do something that can makes them in trouble. While they are using the internet, they can position themselves as a good user because they will responsible with what they do. Teenager will have their experience in many things and if they can share with their parents and gets explanation for what they find, they will not take a wrong way to enjoy their life.
what you said is right, surely parents will give positive direction to their children because it is impossible for parents to give negative direction to their children, if there is I don't think they are good parents. For example, when children suddenly ask questions about gambling, I myself will answer them honestly by explaining everything in detail and directing them not to be reckless with things like this, as you said with gambling that has risks, then I will explain it. For example, we take one country with its current situation, I am sure there are many teenagers who know and even do online gambling because of the free use of the internet and their association with themselves who have an interest in trying it and not a few teenagers are involved in problems because the impact of gambling they do is too excessive.