NEW YORK (CNNMoney.com) -- Identity theft isn't only for adults anymore. Kids are having their identities stolen more and more often.

For children under 18, the number of identity theft complaints reported to the U.S. Federal Trade Commission increased by more than half between 2003 and 2004. There were over 11,600 complaints last year.

You may not even think to get a copy of your child's credit report. But just because your 6 year old doesn't have a credit history, doesn't make them safe from identity theft. You can be a victim of identity theft the second you have a Social Security number. Thieves just create new credit accounts for child victims.

"They start small, and build the child's credit," says ID theft expert Todd Davis.

They may not be able to make that lucrative one-time hit, but often child ID theft can take years to uncover and that means the damage will build up over time, ruining your child's credit even before they hit college.

If you are told your child doesn't have a credit report, that's actually good news. It means that there has been no illicit activity.

If your kid wants to participate in after-school sports, you'll probably have to show a copy of their birth certificate or Social Security card. Put these papers in a sealed envelope and write your name across the sealed flap in colored ink so you can tell if it has been opened.

Initial the back of each page in colored ink so you know if you received the original documents back at the end of the season. And of course, make sure the documents are stored in a locked box during the season.

While ID thieves get a lot of their information from dumpster diving, they can also troll the Web. You'll want to make sure your child's profile on social networking sites like Myspace or Facebook doesn't have any personal information that could lead an ID thief their way.

While your child may receive pre-approved credit card offers, you should be on the lookout for other signs that someone is mis-using your child's identity. Look for unusual marketing solicitations. That means if your six year old is getting Harley Davidson catalogues it's time to do a credit check. Other red flags: your kid is getting notices of traffic violations or bills sent in their name.

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