The issue with canned vegetables is not so much what's IN them-- most contain only the vegetables, water, and some salt. (A few do contain chemical agents to preserve color or texture.) The real issue with canned stuff is what's NOT in them. The canning process involves subjecting the food to very high heat under pressure, which can destroy the more delicate vitamins and antioxidants. Furthermore, canned vegetables have often been stored, even before you buy them, for a long time, which is also not helping to preserve nutritional value. A child fed canned veggies is missing out on many of the most important nutritional benefits of eating vegetables in the first place. And finally, the taste and texture of canned foods is different from that of fresh, and kids develop a taste for what they're used to, so that it might be hard to get them to eat fresh stuff later on if they're accustomed to canned.
I think it's fine to use canned vegetables, sometimes, for little ones. I don't think a steady diet of them is a good idea.
There are also sometimes issues with the cans themselves, with leaching of toxins from the materials used to make and seal the cans. I don't know too much about that, but I'm sure somebody else here does.