the breakfast cereals = хлопья - just generic term for all this stuff in boxes and packets (which mainly implies the way they are made and sold, not what what they are made of, they can be made of anything)

cereal as a technical biological term - злак, and all derivatives

as for popularity of 'breakfast cereal' in Russia - it is more popular with children and younger people than adults (probably more popular with femals than males), and my guess is generally it's not as popular here as in the US. The common stereotype would be that it's not real food, so some stomach centered people would consider it not a substantinal meal.