Sounds like your off to a good start, now time to get your arms wet. Pretty cool that you're getting into this with your son, mine just turned 3 this month. I can't wait until we set up OUR first tank.
A few things:
Dechlorinator, if you're on municipal water. I like Seachem Prime but there's lots out there. I have well water and still use it out of habit.
Thermometer. important as numbers on the heater prob won't be accurate
Holey Rock. A few go by this name, some will raise pH and hardness which may be undesirable depending on what you choose to keep. You can test by putting a little vinegar on a piece. If it doesn't fizz, it's inert
Fish. Why no tetras? there's so many, ruling them all out might be limiting down the road. I like that you two are researching together and making a fish list. Sounds fun. I would recommend going with very hardy, preferably even tank raised (captive bred) fish until y'all get your routine down and the tank stays stable. Nothing will discourage a kid from the hobby like dead or sickly fish. Zebra danios are hardy and active for starters.
I would also go with a fishless cycle, do a forum search, there's tons of good info here on how to do it.
API freshwater master test kit. Testing a new tank may be fun for your son, and it's important to know when it's safe to start adding fish to minimize loss. Mixing the liquids, comparing the readings, and charting the results will make him feel like a scientist. And he'll learn about your fish's water which will hopefully add more appreciation to the finished product (...although it's never a finished product, haha)