According to them, the reason behind this is they're confident that children know already about the money.
Children
don't know about the money. But even if the children don't know about the money, they may still get another bad impact from playing gambling repeatedly. They will make gambling be their habit because playing on it regularly. It is not a good thing for children, they can be addicted too early to gambling. Addiction from a very young age will be hard to recover, this will harm their future.
Yes, addiction at an early age might get continued throughout the entire life and the young minds might go unproductive as the mind won't think anything out of gambling. These days such kind of young people are very low in number. They always have their limits set, and the same makes them consider it an entertaining and fun source rather than losing big and getting into frustration.