Most online services and Internet providers allow parents to limit their children's access to certain services and features such as adult oriented "chat" and bulletin boards. Check for those when you first subscribe. In addition there are now programs designed specifically to enable parents to prevent children from accessing inappropriate materials on the Internet. These tools, while not foolproof, are useful for helping parents control children's access, but they cannot take the place of parental involvement and supervision.
The best way to assure that your children are having positive online experiences is to stay in touch with what they are doing. One way to do this is to spend time with you children while they are online. Have them show you what they do and ask them to teach you how to access services.
While children and teenagers need certain amount of privacy, they also need parental involvement and supervision in their daily lives. The same general parenting skills that apply to the "real world" also apply while online.
If you have cause for concern about your children's online activities, talk to them. Also seek out the advice and counsel of other computer users in your area and become familiar with literature on these systems. Open communication with your children, utilization of such computer resources, and getting online yourself will help you obtain the full benefits of these systems and alert you to any potential problem that may occur with their use.