"Here, we've only got one rule: Never ever let it cool! Keep it cookin in the pot, You've got- hot choc-o-lat!" - Polar Express
I've been using hot tap water to make hot chocolate for the boys. I just add the hot chocolate mix to the hot water, stir and serve. It seems to be just the right temperature and they can drink it pretty much immediately. I'm sure this would vary depending on how hot you have your hot water heater set, but it works for me!