If you add boric acid (roach powder) to methanol (petro dewatering agent) you
get green flame. Its very cool and I used it last Halloween to amuse visitors.
If you make charcoal you can make black powder. A coffee grinder or mortar and pestle helps. Also, you will learn about the importance of moisture and pressure in the quality of the powder. This is not an explosive, just a propellant, unless you confine it. In fact its just a campfire starter if its slow. I actually made
charcoal from plants I grew in the backyard, and the suburban gas fireplace.
Long ago, everyone knew charcoal and its variants, but nitre was tough, made from natural deposits. Now, charcoal making is a lost art but nitre can be bought pure from Home Depot, as stump remover.
If you mix magnetite sands (mined from the beach with a magnet) with aluminum powder, you can make thermite. You can grind the sand in a coffee grinder too.
Always make only a small amount, wear eye protection, use long bbq lighters.
Copper sulphate can be bought from Home Depot as root killer. Mixed with eg baking soda you get copper carbonate, a nice greenish pigment. Other salts are similarly blue or green. They stain porous materials eg plaster quite well. Clorides, acetates, etc are easy too, using common household materials.
None of this impresses my 9 year old terribly, he probably thinks every dad does this.
My mom had a chemistry set, which is pretty amazing. So did I as a kid.
There are other things you can make that go boom, for real, a brisant boom,
involving iodine and ammonia, or silver nitrate and acetylene, or even peroxide and acetone. But those would be dangerous (in various ways) to make.
I've also made yellowcake from uranium ore that I got from the desert.
(And used geiger counters to track it.)
There are more dangerous chemicals under the sink, or used for pools.
Car accidents kills more kids than any other cause. Yet folks drive their kids to trivial things.
I've tried making guncotton but I don't have (and haven't distilled) nitric acid and so it didn't work.
Amateur science is very important. All the STEM (sci, tech, engineering, math) hype won't work unless kids are excited. When I 'retire' (an obsolete, 20th century concept) I might teach science. And/or computer science. Meanwhile I'm a practicing engineer with unusual hobbies.
Speaking of which: out on Trabuco there is a model-airplane flying club, the Trabuco Flyers. I've paused to watch their aerobatics, very cool. They use nitromethane, an explosive (if done right) to fuel airplanes with 2m spans+. The horror!
Heck, even mountain biking is dangerous, I know a guy whose mouth was wired for a month because of a biking accident. And I've known someone who died in a private plane crash. And how many kids break bones, get brain damage, die of cardiac failure, or heat stroke, from "sports"? Before modern medicine, a broken bone might well be a death sentence or at least a lifetime crippling event.
Oh, and I've made x-rays using ignition coils and vacuum tubes. Even taken pictures with them.
And measured the density of gasses using an alpha-particle source and mica-windowed geiger counter.
And graphed the decay of radon using (freely distributable, open source) software I wrote, combined with a 1960's yellow civil defense geiger counter.
Kid injuries so far: chipped tooth from a friend in a swimming pool.
