Priced an archery target lately? They're not cheap--unless you make one yourself.
All you need is a burlap sack (buy one online or pick one up for nothing at a grain elevator or farmers' market) and enough plastic shopping bags to stuff it full (four trips to Walmart in my family). Once that's done, tie the sack shut with a length of twine or wire. A couple of shots of spray paint serve as aiming points.
The target will handle just about any type of traditional bow and modern compounds up to about 45 pounds, making it ideal for kids. The more tightly the plastic bags are compressed in the sack, the more stopping power your target will provide.