Not Coke
Carbonated beverages are full of sugar and probably one of the unhealthiest things you can put in your body. It’s also terrible for your teeth which is why while adults can still get away with it once in a while, kids really shouldn’t be drinking it at all.
Of course, if they see you drinking a glass, they’d want to try some too. When this OP’s child asked them for a sip of their Coke, they just told them that it was black water and that they wouldn’t like it. Again, this is one of those lies that won’t work forever but we’ll take it.