That is a great question. There are some very good online resources for children out there. Let me give you a few suggestions.
First, if a child wants to get better at Math, there is probably nothing better than IXL.com. It not only monitors a child's progress and gives tutorials, but it also makes learning fun. Many schools in New York City recommend this program. The only downside to this is that you need to pay for it. Also I should note that it only goes up to 8th grade math.
Second, there are also free online tutorials. The best general website on practically all subjects is www.khanacademy.org. An MIT professor started this online initiative and it has grown tremendously. In fact, Bill Gates is also helping to fund it. My son goes on this website all the time to learn how to do virtually all things. What is also great about this website is that it is taught by some excellent teachers.