Social Security numbers of children, even newborns, are for sale online, sometimes commanding hundreds of dollars apiece.

“There’s nothing more precious than an infant’s Social Security number,” consumer advocate Adam Levin said.

Levin said thieves consider children’s Social Security numbers as the most valuable because they may be able to use the numbers for years without anyone finding out.

“No one’s checking, and no one is going to check for some time,” he said.

Here are some warning signs that a child’s identity may have been stolen:

Such letters may seem amusing at first, but Levin explained that they indicate that a child had a credit file, which he says is a big red flag.

Levin added that if the IRS sends an unexpected tax bill, it could indicate that someone other than the child’s parents is claiming the child as a dependent.

Levin said that if it’s believed that a child’s personal information has been compromise, contact the three credit reporting agencies and put s credit freeze in place.