What percentage of people like chocolate milk?

I am not sure that all kids do-- it's not available in every country. But many kids from the US and Canada certainly seem to like it. The reason has less to do with chocolate than it does with sweet flavoring-- studies have shown repeatedly that babies like tastes that are sweet. Chocolate flavoring adds sweetness to milk; so do other kinds of flavored syrups, like strawberry, coffee, or banana, which are also popular with many kids.