Hi, this sounds like really great start, and a nicely stocked tank!
Guppies and Endler's Livebearers are closely related, and can interbreed, so hybrids between the two are commonly found in the hobby. In fact the guppies I adopted appear to be guppy/Endler crosses, judging by their size and body markings. They'll be absolutely fine together.
Personally, I wouldn't add any schooling fish, since depending on species, they need groups of 8-12, and you'd be approaching overstocked if you added them to your current stocking. But you could easily add a few more male guppies and/or Endlers! Could take your son to the store and choose some males with different colours that he likes. Just make sure to examine the tanks in the store and make sure the fish in the store look healthy. We can give more tips about choosing fish in the store and about quarantining them for disease if you'd like.
Only other recommendations I'd make is to up the amount of water you change weekly. Remember that the solution to pollution is dilution, and if you're removing 25% of the water, you're still leaving 75% of the old stuff in there. Over time, with smaller water changes like that, levels of nitrates can gradually climb week by week, and lead to old tank syndrome. 25% isn't terrible, especially since you're currently not overstocked and have fish with a fairly light bioload. But changing 50% weekly won't do any harm at all, and will improve water quality in the tank.
Some people are wary of large water changes, but clean fresh water is the best for fish. Just make sure to temperature match the new water to the temp of the tank before adding it, and use a water conditioner to declorinate the water before adding too. Use a gravel vac to clean the substrate during every weekly water change to remove dirt, debris and uneaten food - especially when a kid is feeding the fish, it's very easy to overfeed the tank, since fish need a lot less food than we tend to give, and overfeeding can also decrease water quality, so the maintenance of gravel cleaning and doing decently sized water changes are essential to keep the water quality good, and the fish healthy.
The only other tips I have are to add a background if you don't have one yet, plain black construction paper will do! Helps the fish feel safer, and also makes the fish colours and plants pop.
Lastly, the plants. It's great that you have the frogbit - floating plants are great for helping the fish feel safe, and also great for sucking up ammonia that the fish produce quickly. The other plants are brilliant plants for beginners to the hobby, so good choices! But they're slower growing, so don't help as much with water quality. So personally I'd add another fast growing plant, ideally one that will grow tall and provide breaks in lines of sight for the guppies and endlers, and provide hiding places at any height in the tank.
Plants I'd suggest are limniphilia sessiliflora - beautiful, fluffy and easy to grow plant, fish love it, and because it grows fast, it's good for sucking up ammonia and improving water quality. One of my favourites, so I often recommend!
The right aquarium plants
tropica.com
Worth watching the video, it'll grow to the full height of the tank, and it's easy to maintain. Once it's tall enough, you can just snip it with scissors, then replant the trimmings and they'll grow too. I once propagated it like this and formed it into a sort of hedge that hid the sponge filter by just looking like a dense bunch of plants, and both fish and shrimp love it as a hiding place.
You can also do the same with hornwort or elodea, but they're not quite as pretty, and hornwort has a habit of dropping all it's needles if it's unhappy, which makes a mess to clean up.
None of these are essential, they're only suggestions, but it sounds as though you're doing great, and it's a lovely thing to share with your son! And a great way to teach him about nature, and responsibility.
