This is from Miami Children Hospital's magazine in Volume 9, No:2
"The incidence of kidney stones in children has been increasing steadily in America over the last decade..."
The first reason mentioned in the article is Salt! The article reads:
"Excess salt is contained in many foods that children love and eat now more of than ever before, such as french fries, cheeseburgers, and potato chips. In addition, with today's prepackaged, quick-meal mentality, parents may not be aware that many convenient foods such as lunch meats, packaged meals, canned soups, and sports drinks, contain high levels of hidden salts."