Parenting Conundrum

On Shabbat morning, my son came to me with a request. It seems that a group of his friends decided to get together and play poker on Shabbat, and they were going to play for chips. Potato Chips. Could he play?
I found myself in a very sticky situation. On one hand, it's potato chips. The kids are playing a game together. And more importantly, I already say "no" to a whole host of things. For exapmle, we don't play ball on Shabbat. His friends often do. So I truly felt that I didn't want to give him another "no."
On the other hand, I really did not approve. I know that the game itself is harmless, but the gambling bothers my. Why should kids need to gamble - even potato chips - to have fun on a Shabbat afternoon? Can't they find something better to do than poker?
In the end, I tried to play it both ways, which I think was the proper way to go. First, I explained to him the halachic issues with regular gambling. In a nutshell it goes like this:
The Gemara says that when a person bets, he doesn't really think he's going to lose. Actually, he rather hopes he's going to win, because if he knew that he'd lose for sure he'd never have bet. So the winning of a bet is called "avak gezel" - the dust of theft. It's not outright theft, but it's certainly not honestly gained winnings.
Then I said, "While I know that you'll only be playing for potato chips, I don't think that the activity of gambling is a healthy and productive activity. It's not a good habit to get into. But I'm not going to tell you that you can't. I'm only going to say that I don't want you to. It's up to you to decide what to do."
What do you think happened?