Have them keep track of their progress in a log. If you choose, you can keep things honest by having them record all transactions with a neutral party, such as yourself. At the end of a given period of time, have them write up how much they made or lost in the stock market. There are great opportunities in business, and great risks. For every person who gets rich in business, there are dozens of others who just get by, or who fail.
People make it in business with smart ideas, lots of hard work, and more than a little luck.