I suspect some parents let their kids have whatever they want just to shut them up. After the precedence is set it's hard going back. Ask me how I know.
My kids are too young to even want to try coffee. I let my 6 year old taste the crema from an espresso and she scrunched up her face in disgust. That should take care of the situation for a few years.
My brother-in-law's 10year old boy has a fondness for iced vietnamese coffee. I suspect it's the sugar from the sweetened condensed milk that he likes. They let him have a sip or two about once a week when they go for vienamese noodles.
None of my kids, nephews or neices drink soda more than once a month, if at all.