Most pet stores offer frozen fish food, and some will have live food. Usually insect larvae (bloodworms, glassworms or mosquitos) or brine shrimp are offered. They do seem to enjoy them frozen for a treat, but if you can get some live it's really interesting to watch them hunt. The bugs go fast though once they're in the tank.
Do these tetras need salt? Some fish do and some don't, so I'm not telling you not to do it, but make sure you know what it's for in this case.
You didn't say much about what else is in your tank setup, but even the biggest fish appreciate some cover, even if it's just plastic plants, so that they can sort of hide and feel safer. It isn't necessary to buy the big expensive decorations at pet store prices, but you can if you think they look nice. Some are really cool.
Or you can make them something. Live plants are great if you can take care of them, but with a 2 year old it's probably more work than it's worth. Driftwood can be good too, but make sure you read up on its preparation and be aware that it will color the water for a long while. I've also used brick, which I think looks good. Home Depot or similar charges just under 50 cents apiece, and if you get the red brick with holes in it that's sold for landscaping it makes great little tunnels for the fish to swim through and adds to the interest of the tank I think. None of that is necessary, but it can be fun.