It depends if gambling is very rampant in that country, but if it's about accessibility to any gambling-related sites or tools it should be the parent's obligation to see to it that their children will not be addicted or the subject is properly explained to children, the parents can teach addiction to their children effectively, because they have a moral obligation to their children to teach them the harmful effects of gambling.
In some countries, Gambling is accessible to teens but in most countries, the legal age is 18 still teens gamble because they have access to the internet or they have friends who are into gambling. Parents surely have a duty towards their children but all they can do is monitor and keep a check. They can guide them but they cannot be with the child 24/7. I think since most part of the day a child is in school therefore it becomes a moral obligation for them to also inform the about the harmful effect of Gambling.