Nothing wrong with it really as long as it doesn't get out of control. Back in the 80s before we had the net and offshores I was betting with my friends and playing blackjack at lunch, on the bus for field trips, and for half our journalism class. I would say at least 25% of the males were taking part in it. With the poker boom and the way sports betting is so much more accepted today I would be stunned if that number isn't closer to 50% now.
In any case no big harm in having kids gamble as long as the stakes aren't hurting anyone. Those kids will grow up and be bombarded with gambling options every day not too far into the future so they might as well get used to it and see gambling isn't all that much to get crazy over. If you try to keep them from doing it they will just want to do it more and will be far more likely to have problems with it as a result. Sort of like how many of the kids whose parents let them drink at 15 turn into alcoholics? I would guess its less than average because they learned to do it responsibly and with supervision.