Did no-one else have children who, behaving like the young apes that they are, and learning by copying what is good to eat and drink and what isn't, asked (not necessarily in words; we have a photo of Minibaggs as a toddler sitting with her dad on the settee, with half her hand in his champagne glass. I forget what the occasion was and he had his free arm around her, and didn't even realise till after I'd taken the 'snap') to try a bit of whatever?
A sip here and a taste there isn't going to harm a child.
But then, I'm a bad mother who gave my kids up and over mocha sponge pudding several times in their growing up years. It never occurred to me that the small amount of caffeine in it would do them any harm. And, do you know, it didn't.
I also gave Minibaggs very milky tea when she was wee. It was the cows' milk that caused her (minor) problems, not the caffeine.
Some modern parents need to chill a bit. There are plenty of things they need to protect their children from. An occasional sip of granny's coffee isn't one of them. As with illegal drugs, the banning and policing of things like caffeine, can cause more harm than actually being a bit relaxed about them.
The yelling and screaming scenario that the mumsnet OP was about is far more likely to encourage screwedupness.