Sake contains a high concentration of amino acids, which are responsible for giving foods the taste of umami. This is what gives authentic Japanese teriyaki sauce it's depth of flavor. Since the sauce is boiled until it is significantly reduced, nearly all of the alcohol will burn off, so it should not be a problem for children. If you are concerned with residual alcohol in the sauce, consider this. Alcohol is a natural byproduct of fermentation, so most things that have undergone some fermentation, such as ripe fruit, yeast-leavened bread, soy sauce, and miso, contain some alcohol. If you absolutely cannot find it, the best substitute is water. It will not be quite as flavorful, but it is a better substitute than fruit juice, or wine, which is acidic and will make the sauce taste sour.