IRS tax returns could probably help reunite hundreds of kids a year with their rightful parent, bur privacy laws forbid the agency from helping investigators. As the New York Times explains, even people who abduct kids in custody disputes sometimes take the odd step of declaring them on their returns.
The privacy laws contain some loopholes, but they don't allow the IRS to divulge the whereabouts of the kids because they're generally part of local and state criminal investigations, not federal. As to the question of why the abductors would put the kids on their returns: “It doesn’t make a whole lot of sense,” a retired FBI agent tells the Times. “But if they were thinking clearly, they wouldn’t have abducted their child in the first place." (More IRS stories.)