Parents can at least set their mobile phone protected content, privacy, and payment methods, so the kids can not use for something that their parents don't know. The parents need to explain that playing gambling is not good for them because once the kids become addicting, they can use their parent's money to gamble. The parent's control always needed to prevent the kids from browsing on the gambling or the other thing that they should not visit in their ages.
I agree, controlling the money is a good way to enforce strict no gambling rules. But there are also a lot of apps having casino games with play money. And the kids could still use their monthly allowance to gamble. It's better to supervise the kids more closely in my opinion. I remember when I was younger I was using various apps which where instantly added to my monthly phone bill. So my parents didn't really notice at first where I was spending my money. And only after a some months they noticed.
If those kids download those games, they will slowly become addicting because I am sure that the games will be attractive, and they will play repeatedly. I am a bit worried if they use their monthly allowance to gamble because that can make them addicting too. There is no other way that we can do except explain to them how dangerous gambling is for kids to think by themselves, and they will not try to play gambling.