Some of my neighborhoods are playing gambling in front of their children and guess what, I'd see their kids playing with them already. We thought that educating them earlier could help them understand what gambling is all about but the results are too opposite, we are grabbing them to gain curiosity and try.
Yes, there is a right time for them to learn about it, and when they are already at an illegal age where we believe that they are capable enough to understand the whole concept of gambling.
From your post I have found that there are already two type of gamblers - adults and kids in your surrounding. The difference are - when adults gamble, they do it for two reasons, have fun and earn if they are lucky. Kids gamble in different way. They gamble for fun, for small things, trifles. I've never heard that 5yo kid was so into gambling, that he has lost parents house and now owe to half of the city. Never an adult play against kids seriously, nor accept their unrealistic bet. Never adults gamble against kids to win all their money, make them broke and etc.
If someone is afraid, that his kid will watch them play, then take parents money and gamble them away, then the problem is not in gambling, but in education. It is parents fault that they neglected the child, did not explain what is good or bad, what is appropriate to do, and what not.