When I was working in the motion picture industry, we sometimes used a "Dulling Spray" to reduce the glossiness of objects we were filming or which were in a scene we were filming. This spray came in two versions, permanent and removable. I noticed that a removable version of this spray is still available at a pretty inexpensive price.
http://www.filmtools.com/kryldulspray.html
If you are usig studio lighting, you could always go with a circular polarizer as well, though the dulling spray does quite well by itself.
Using this spray on the glossy cereal box, MIGHT JUST make the lighting of these boxes easier.
Caveat: Although the spray is advertised as "REMOVABLE" I would be quite careful spraying it on anything that is valuable or non-replaceable such as artwork. Of course, the cereal boxes would be considered expendable.