J.L.
More than anything, it's the BPA lined cans that pose the most health risk when it comes to soup, not sodium. There's lots of debate out there about it causing hormone problems and more in children. Next concern would be sodium and additives.
Try homemade, or glass jarred soups or organic dry soups and see if she likes those as much as the Campbell's if you're concerned about the quality of the soup.
For more on BPA canned goods see:
http://www.consumerreports.org/cro/magazine-archive/decem...
http://abcnews.go.com/GMA/OnCall/bpa-dangers-pantry/story...
For BPA free canned goods see:
http://blogs.mercola.com/sites/vitalvotes/archive/2010/03...