If you're a kid, you might be 'allowed' to add soy sauce to your rice, but as an adult, it is seen as a low-class thing to do. Or, some see it as something a Chinese person might do (as I have been told).
Instead, Japanese have a solution ...
furikake in all sorts of varieties and flavors including ones for kids. They come in small individual-sized sachets or small shaker jars that you can use to sprinkle on top of your rice.