Even countries where gambling is legal should ban ads on TV because it's not unknown by the authorities that kids watch TV as well, and this thing can have a pretty bad influence on them if they see those gambling and casino advertisements. I believe some countries such as Australia have taken such steps and have banned gambling advertisements both on TV and online spaces which are social media and other locations on the internet.
If it’s legal and allowed in a country, why still should there be a ban for casinos to advertise and make themselves known to the public? Also, I don’t think gambling ads have that much of an effect on young children as you seem to think. They’re more likely to ignore such advertisements as the ads aren’t designed as they’re aren’t intended for young minds.
The responsibility of raising kids right rests primarily with the parents. People won’t just give up legal activities they enjoy because it could have a bad influence on your kid. If you as a parent dislike certain activities and advertisements cause you think it might influence your kid, it’s on you to make adjustments to suit your preferences for your kid and not to clamor for its outright ban.