I happen to like Jamie Oliver; he has a bit of an ego, but his heart is in the right place.
I am no nutritionist, but I do happen to be an expert on international law as applied to food production and global importation restrictions as applied to genetically modified crops etc.
As is the case with all things, people react poorly to change and abhor the the command and control style of government. In short, even if something is better for someone. . . and they know it. . . they want to make the ultimate choice. Children are not capable of making choices with the same degree of consideration that adults are, they know what tastes good at the moment, even if they routinely get a stomach ache 15 minutes later it is of no concern, they will repeat the process the next day.
There are many healthy foods that can be made to appeal to children, hell, a simple PB&J on whole grain bread is a lot healthier than what many of these kids get. A bright colored salad should naturally be more appealing than a gray unrecognizable disc of meet claimed to be a hamburger, but the kids will pick the burger, often times for social reasons. Following WWII canned goods, microwaveable meals etc. became more popular as they made life easier and became cost-efficient. Preserving foods has historically been a good thing, salted meats, pickled vegetables, but only from the stand point that it enabled people to eat when certain foods were not readily available. Fresh is almost always best. At present factory farms churn out corn, low grade meat and, coupled with artificial flavorings and copious amounts of salt along with a heaping dose of government subsidies. . .feed our children crap. If these products cost what they should and quality standards were not at close to zero, children would be getting a much different school lunch. More importantly, the parents would not be feeding them, and thereby legitimizing, the same junk. Provided with options a child will eventually get it right. In short, people should have the option to eat healthy and be able choose to eat what they want, but they should bear the consequences for those decisions. Therefore we should be helping people make the right ones.