Take half the milk and heat it (on a stove or in a microwave) but don't bring to a boil.
While the milk is heating, chop the dark chocolate and set in a bowl.
Once the milk is hot, pour it over the chocolate and whisk until the chocolate melts.
Add the remaining cold milk and optional sweetener. (You can make top it with warm milk aslo but if making for kids they won't want to wait for it to cool so this is a way to give them warm chocolate milk)
Notes
you can use honey instead of agave for babies over 1 years old