Was I at all wrong to let my son have candy after the woman in front of me in line said no to her own?

So I'm checking out at the grocery store. The woman in front of me has three children, all begging for candy. She tells them no several times. My son overhears the commotion and asks if he can have a treat. It's just fifty cents, so I say sure. He was good through the store anyway. The woman's children immediately begin screaming that it's not fair that my son got candy, and the woman gives me the evil eye. In a snooty tone she says, "Thanks a lot." I hadn't really thought about her position before buying it, but then again I don't think that I should need to. I don't know her. I have nothing to do with her. If she can't spare a dollar to shut her kids up then that's her problem. Am I wrong?