hmm...
My parents never forbade me to drink coffee, moreover my granny used to suggest me a cup from time to time! The matter is that I didn't like it that much when I was a kid.
I don't really think it's always good to prohibit something. You might just explain your children that coffee is good for grownups, but harmful for kids
Or
when they ask for coffee just give them a cup.. without sugar
I think that will turn them away for many years in advance