Lunchables, those ubiquitous plastic packets of cheese, deli meats and crackers that many parents choose for convenience, contain concerning levels of lead and sodium, Consumer Reports warns.

To arrive at that conclusion, the advocacy group tested Lunchables made by Kraft Heinz, as well as similar lunch and snack products from other manufacturers.

"There's a lot to be concerned about in these kits," Amy Keating, a registered dietitian at Consumer Reports (CR), said in a news release announcing the findings. "They're highly processed, and regularly eating processed meat, a main ingredient in many of these products, has been linked to increased risk of some cancers."

While none of the products exceeded regulatory limits, five of the 12 tested expose children to 50% or more of California's maximum allowable amount of lead or cadmium — heavy metals that can cause developmental and other problems in kids, the group stated.

A 3.2-ounce Turkey and Cheddar Cracker Stackers Lunchables contained 74% of California's level allowed for lead and 49% of the daily recommended salt intake for kids ages 4 to 8.

"The kits provide only about 15% of the 1,600 daily calories that a typical 8-year-old requires, but that small amount of food puts them fairly close to the daily maximum limit for lead," said Eric Boring, a CR chemist who led the testing. "So if a child gets more than half of the daily limit for lead from so few calories, there's little room for potential exposure from other foods, drinking water or the environment."

"We don't think anybody should regularly eat these products, and they definitely shouldn't be considered a healthy school lunch," Boring added.

CR has called on the U.S. Department of Agriculture to remove the Kraft Heinz products from the National School Lunch Program, with its petition nearing 15,000 signatures. The federal program provides low-cost or free meals to kids on school days.

A spokesperson for Kraft Heinz defended the meals.

"Many of our Lunchables products are a good source of protein, offering nutrients through meats and cheeses. We've taken great steps to improve the nutrition profile of Lunchables, including recently unveiling Lunchables with Fresh Fruit, in partnership with Fresh Del Monte, and reducing the sodium in all Lunchables crackers by 26%," a spokesperson told CBS News.

"According to current science, processed foods arbitrarily classified as 'ultra-processed' are not necessarily less nutritious," the spokesperson added. "In fact, many processed foods contain added nutrients, providing even more benefits to the consumer. The classification of foods should be based on scientific evidence that includes an assessment of the nutritional value of the whole product, not restricted to one element such as a single ingredient or the level of processing."