I personally consider most things from a can or a package to be junk food. These foods are full of artificial flavours and preservatives, and anything that is was once a natural food has been either dehydrated or bastardized in some way in order for it not to spoil. These types of foods are also unnecessarily high in sodium - generally more than a whole day's recommendation in one serving! I also feel that anything fried is unhealthy.
I try to feed the kids foods that I feed my own family. I know that these prepackaged foods are usually ready-made and much less time consuming to prepare, but if you prepare the lunch from what you served your own family the night before, it's all ready to go!
Having said that, I do believe that everything is fine in moderation. I don't serve ready-made foods or processed meats for lunches, but I do let the dck's indulge in a sugary treat (homemade cookie or cupcake) once every week or two.