There's no reward, you'd be getting the same tax break if the child was not kidnapped. The rule is about how long it takes to lose the tax break in case of kidnapping.
Except if the kid was not kidnapped, you have to pay for the kid's expenses and welfare.
I expect somebody who wants to keep the tax break without the kid is not too worried about the kid's intrinsic welfare.
Who would even agree to kidnap somebody else’s child for this purpose? I’d suspect the first 9 out of 10 convicted felons would still tell the police your plans. Most people would be too suspicious that you’re trying to entrap them, asking them to kidnap your child.
And what can you offer someone to agree to do this if your own reward is a temporary tax break? At this point, you might as well kidnap your own child.