It's hard to say if a child is involved in gambling, unless taken unawares. In as much as the underaged player no longer stays with his guardians, they'll be no way to go around the situation, other than inviting him home and recommend a rehab program or therapy session. Thus, Parents have to deal with this carefully, to retain the sanity of their wards.
Therefore, the player's pocket money should be restricted, or cut into half, such that whenever he asks for more, he'll be required to return home. Afterwards, the kid with addiction can directly be asked questions, regarding what mostly consumes his/her funds. Then, followed up with a professional therapist.
We know that social control is no longer running in many environments, adults who see underage children making mistakes no longer have the initiative to reprimand or remind them.
Interesting you came up with this, usually, it was helpful, but recently elderly people (in my locality) began to misuse the power of social control by complaining over little things which wasn't a huge problem. They failed to realized that young people move on quickly, sooner, nobody listened to such cautions from strangers who are not their parent. I miss those days, but, personally, I didn't like it either when they began complaining about dress codes and all that, it wasn't relevant.
That was the reason it's dying down in recent times. For instance, I asked a friend about his take on such complaints from elderly people, he blatantly told me he avoids people who complains a lot about his appearance and all that. Assuming they focused on addressing addiction and cautioning people who are actually going astray, there would have been some quality respect left in social control.