Welcome to the fun world of nano's. Not sure if you have kept a smaller tank before, but the rundown is that things happen faster in a small tank. You are going to have to make sure that you or your son, keep up on water changes. It sounds like you know about the nitrogen cycle, which is great! Make sure you take your time and let it ride its course.
One of the biggest things about SW, as you prob know, is patience and not overstocking. You can most likely only fit 1-2 small fish in there, bioload wise. Not sure how old your son is, but what kid doesn't love a clownfish?! They are great for nano (stay away from some of the larger species though).
Before adding the fish, you should add a clean up crew of sorts. Most of us like snails (cerith or nassarius) and maybe even a few hermits. Unlike fish, inverts don't add a lot to the bioload of the tank. So with a few different types, like some cool shrimp/crags, you can add a few more things for your son to check out.
Corals - Not sure what type of lighting you have on your tank, I am going to assume PC. With PC lighting, you can keep some cool zoos, shrooms, and other low light corals.
Keep us updated, ask questions, and go slow.
Good luck!