Milk, lactose free milk or almond milk (depending on food tolerances) and chocolate syrup added to taste. Let kids pick how much chocolate syrup to add and mix in. Throw on whip cream and little marshmallows, sprinkles or other candies as desired.
Giving kids milk plus chocolate power plus the latter ingredients is an invitation to an awful mess.
Note: I recommend almond milk because soy milk has a bad aftertaste, coconut milk isn't quite "milk chocolate" flavored for our tastes but is a good option if you like Mounds chocolate/coconut candy, don't use oat milk.