So activating the underage user feature is very helpful and it seems like several people here have answered it.
I agree with the last few words you said that parents must supervise every activity their children do to avoid this bad situation because gambling is currently growing very fast and advertisements everywhere are related to gambling so this supervision is very important and be alert if the child's behavior is a bit different.
Gambling should be kept away from children but the point can be considered completely moot if the children have access to the internet. That is core nature of the problem, really. And we cannot control the internet, we cannot impose real regulation upon it. So what is left to do? As long as they have access to the world wide web, there is no way to effectively stop them from learning about and trying things such as gambling. And there are a lot of worse websites that they have access to.
And we cannot put the blame on parents, because the world is digital. Having a smartphone is the bare minimum of living in this modern world. So there is a limit to what parents can and cannot do.
We need to concentrate on solving this issue.