Is it illegal to claim zero dependents on your job if you married and have kids?

On your return - you can only claim the "qualifying" ones that exist...and want to because they are deductions. On your W4 (work)..as many or as few as you like or need. Remember, it doesn't change the tax you will actually owe, only the amount withheld...if you claim more than you have you it can cause you to have too little withheld, you will not only owe the extra tax, but a penalty and interest will be applied (and it may be an audit flag), all of which can become really substantial. So, especially if you expect to owe tax on income from other sources, using zero can compensate. The w-4 has a worksheet that helps you determone the right number to claim.