If you trust them enough and are comfortable with it a credit card is the best way to build credit as long as they pay it off every month. Explain to them how it works first before they get themselves into trouble (debt). You could start by just having them purchase gas and groceries. As long as they are responsible with that they could use it for every purchases they make, again as long as they pay it off every month. Not only will this build their credit but they can also get cash back on everything they buy. 1% may not seem like a lot but it adds up to thousands of dollars over a lifetime.