That's our responsibility as a parent, to advise, to manage/control and to help him to get out from gambling because he is still underage. For the winning, if it is something that we considered as a huge amount or a life changing amount, I will not let it go so I will help him to withdraw it but once it is withdrawn then I should keep the whole amount. Starting at this point, I will give more attention to what he does.
If the amount won is big, it might be a very complicated situation to handle. But my problem about the situation is the example we parents should set for our children. Wouldn't this child take situation as permission to continue breaking the law? However if there is a means to guide the child to avoid engaging in illegal activities like underaged gambling, maybe the parent might help cash out the funds. You also made a good point by advising parents to keep their eyes on such a child because he need to be guided accordingly.