Sue
Everything is toxic. Dilution is the solution to pollution.
Cardboard is paper. The papers are clean enough for the food industry. The inks are likely to be the worst of the chemicals, and many inks now days are soy based to avoid the toxicity problem. The amount of ink on a piece of cardboard is not likely to be a problem.
I'm not sure what glue is used to hold the paper layers together. Since most box companies produce boxes for many industries, including the food industry, they are pretty tightly controlled on what kinds of glues they can use.
Most box makers print their name somewhere on the box. Most box makers are national. It is likely that you have a producer near you. If you are worried, call them.
I say your risk is minimal, use them.