Someone, whether it's you or the guardian, needs to speak up immediately.
I was at the grocery once and saw a woman allow her young son to choose a doughnut from the self-serve display and eat it while she finished her shopping, probably because he was restless and bored. (I noticed them in particular because I was going to treat myself to a doughnut, and the little boy touched several with his bare hands before lighting on one that he liked, but that's a different etiquette issue.) I ended up in the check-out line behind them, and it was pretty clear that she didn't tell the cashier that her son had consumed a doughnut while in the store. So, after she was done checking out and I was able to get to a place where I could speak to the cashier directly, I asked if she'd paid for a doughnut. The cashier wheeled, sprinted after them, and caught them just as they were about to exit the store. A manager saw the kerfuffle and headed that way too. The end result was the woman came back to the checkstand and paid for one doughnut, giving me the stink-eye the entire time. But, I hope that she and her son both learned a lesson that day, and that they won't do it again.