The giving spirit is within most people and helping a child is one of the most compelling needs we can answer as humans. However, there are a number of less than worthy child charities that are looking for sponsors as easy marks for money instead of actually sponsoring a child. In many cases,neither the charities nor the children actually exist.
How can you avoid being scammed? Do your homework before you commit your time and money. Don't take your best friend's word for it that "XYZ is a great child charity". Make some calls and look up the charity on the Internet. If you find a bad review, investigate that as well and make sure it is a legitimate complaint or information regarding someone's experience with the sponsorship. Remember, sometimes bad reviews are posted by disgruntled ex-employees who have a different motivation than telling an honest story.
Make a short list of the websites and information you find and if you find enough negative information, tell the charity that you'd love to help but you'd like to know a few things and bring up the negative information. A charity with nothing to hide will address the issues and answer all your questions. If the child sponsor is a scam, they will most likely hedge and dodge your questions and won't give you much to go except most likely assuring you, "Everything is just fine" as they ask for a check from you.
Last but not least, what is your "gut feeling" about the place. Nearly everyone who has been scammed will tell you later that they felt something wasn't right. Do you feel that way? Proceed with caution and do more research if so and you'll probably avoid being a victim of a child sponsor scam.