I'm with Clint, I would've offered to buy the cough syrup for the kid. While it feels good momentarily to confront neglecting and abusing parents, you have to remember that after they leave the child will be alone with them. If you make an abuser mad and shame them in public, they very well might take it out on the child when they get home. It's probably better to be "helpful" ("Oh my goodness, let me pay for that cough syrup, I know how it is when they're sick and you're running low on cash") than confrontational when you see an adult mistreating a child in public.