hmmm...Since the soy is fermented (so is partially broken down or "digested"), I have always heard that it is much more easily dealt with in the body than, say, fake-meat processed soy items or even soy milk. So I would be surprised if small amounts of soy sauce would be causing a problem with your nursling.
As for the fish issue, if you get a kosher brand there will be no fish products in it (both Kikkoman and LaChoy are kosher, as are some of the better major brands of Soy Sauces, Shoyus and Tamari sauces at your local health food store. Look on the packaging for the symbols: U-inside-a-circle, or K-inside-a-circle, or the K-inside-a-star...those are the most widely seen kosher symbols). As long as it doesn't say "fish" right below/beside the symbol you can be absolutely certain that fish isn't in it.
good luck!