There is an age that a child will reach in life where, even if you don't teach them about a particular thing, they will either come across it in one way or another, either through social interaction with friends or on the internet. They will definitely have to learn about it.
Of course you are right because most people especially some parents always misunderstood the whole idea of educating there children about gambling because just like you observed in as much as is very important to teach our children things they shouldn't venture into but we should also consider there age because there is no way they can expect a child of 5 years to still remember what he was taught when was five.
So perhaps a child with that age should only be well monitored and guided so that he will never be influenced by anything and perhaps when he or she has gotten to the right age then the parents can now educate them knowing fully well that if they failed to teach them they will eventually learned it from outsiders which is going to be very bad for them because they wouldn't really understand how to control there emotions towards it.