Sounds like you really did your homework, and you've been doing a wonderful job with the stocking!
Seriously, kudos, a lot of people don't, the tank fails, child is upset, and it puts them off the hobby because it went wrong, but you've already gotten over the hardest part, and sensibly stocked, I love to see it!
I'm sure you can began to add more fish, or add a school of tetra or rasbora (but in higher group numbers than 5, they do well with more in a school than less, and how many would be best depends on the species.
The tank stocking, especially if well planted with some fast growing live plants, can handle the numbers, the only notes of caution/advice I'd suggest first, is to get a little quarantine set up going first, so you can isolate the new fish and make sure they're healthy/medicate any problems, before adding them to your main display tank. Quarantine is so important, but often skipped until people learn the hard way, and since your set up is doing well, this is a great way to make sure it stays that way, since it's very easy to introduce illness and disease when adding new stock from a store. QT set up doesn't mean you have to have a second tank running full time, can be a relatively small tank (a 10g tank is usually plenty big enough for a QT/hospital/emergency tank for the size fish you'd be stocking), and can be easily set up just for the time you need it, then put away at other times. People here can give much more detail about how to make a quick and easy QT set up for when you're buying new fish, or need to isolate some fish for an illness or anything.
Second tip would be to get a liquid testing kit, to test your tank water for ammonia, nitrites, and nitrates especially. The API master test kit or NTL labs test kits are good, and the liquid kits are more accurate, last longer and get a lot more tests out of them than the dip strips. But then you can actually test the numbers for your tank, see how rapidly (or not) nitrAtes are raising, and whether it's a good time to add more fish, or once the new school is added, whether the system is handling the new additions, since the added bioload can take time for the nitrifying bacteria to adjust to the extra waste being produced, and cause a rise in ammonia, it's good to be able to test those numbers and know for sure what's happening with your water.
Especially when you're starting out, it's useful to be able to test your tank water regularly at home, so you get a feel for how often you should be doing water changes, whether you need to increase water changes from once to 2x weekly for example, such as when breeding fish or stocking higher levels, higher bioload means more water changes are needed, you know?
Also very useful to be able to test the water when things go wrong, as they inevitable do from time to time in this hobby I'm afraid. Being able to test if it's an ammonia spike, or if nitrates have gotten way too high, can let you know what's happening in the tank, and how to fix it.
Hope some of that is useful, congrats again on such a successful start, and lovely to be able to share this hobby with your son! A big welcome to the hobby and to the forum to you both.
Whichever school of fish you wind up choosing, I'm sure they'll do well since you're prepared so well, and any of those tetra or rasbora species in a good number (12 or more, ideally, and I think you could do either of those in a 20g with that current stocking, or choose another similar rasbora or tetra - cardinal tetra are even more beautiful than neons, IMO!) but any of those schooling types will add activity, movement and colour to the tank that will work well with your current stocking too.
I'd also consider adding just a few more pygmy corydora, at another time from the tetra/rasbora. 8 is a great starting number for pygmies, don't get me wrong! But they do even better in larger groups, being a shy, small fish, and will be even more active and feel safer in a group of 12 or so. They have a light bioload, so plenty of plants for them to hide in, the few male only endlers and guppies, plus a schooling group of tetra or rasbora, and that sounds like a lovely, fully stocked tank you and your son can enjoy and watch thrive for a long time.
it's worth noting your water hardness too, guppies and endlers are hard water fish whereas tetras tend to be soft...it's much easier to stock fish that suit your local water rather than try to adjust your water to suit the fish
