the other posters are correct - true Martian soil contains perchlorates, high levels of iron, and can be highly toxic. What you want to look for is called "Mars Regolith Stimulant". There are a few websites that have recipes.
The five most abundant ingredients, account for almost 90% of the dirt taken from Mars samples.
- SiO2 - 49.5%
- Fe2O3 - 17.9%
- Al2O3 - 7.2%
- MgO - 7.7%
- CaO - 6.7%
That seems like a good starting point. If I pad those out to reach 100% and use the results as a weight ratio, it should make a decent first batch of home-made Martian soil. Luckily most of this stuff can be found in hardware and health food stores.
Silicon dioxide, yeah sand. You can go budget and get a bag of propagation sand (it won’t be 100% silica). If you want a bit more precision you can hunt around for educational or scientific samples that contain less impurities. You can get 2.7 kilograms for about \$16
Iron oxide, is red cement colouring and is often advertised as red oxide or similar. You can get 2.2 kilograms for about \$20
Aluminium oxide, is used as an abrasive. It gets stuck to sandpaper and is used in sandblasting. It was a bit difficult obtaining smaller amounts in Australia (places wanted to sell me 20kg bags). You can get 340 grams for about \$10.
Magnesium oxide, is a dietary supplement found in health food stores. You can get 225 grams for about \$10.
Calcium oxide, Now this one is tricky. I couldn’t easily buy calcium oxide. It seems that calcium oxide reacts with CO2 in the air and gets converted into calcium carbonate. But you can buy calcium carbonate (CaCO3) as a dietary supplement. This can then be turned into calcium oxide by ‘lime-burning’, just heat it up in a kiln to above 825°C. You can get 340 grams of calcium carbonate for about \$10
and others that lets you buy your own (I won't link to them because of advertising, but the Martian garden is one such site). I think you could get close with the above recipe.
In any scenario, I would strongly recommend supervising your child when they are working with the stimulant. It should be handled with gloves and a respirator mask. Ingestion is very dangerous, and it might not be the worst idea to check the number of your local poison control center. While this may seem like overkill, it would make a great section of your child's science project. "Methodology", "Safety", and "Standards & Practices" are all very important parts of working in a lab and in engineering.
Best of luck!