As an additional thought:
What you claim on her W-4 really depends on how much income she is expected to have from that job, and you have not indicated that. The nature of jobs that a 6-year old could have might be fairly minor $$-wise, but for child models and actresses, their pay could be substantial. So pay attention to the limits that DoninGA noted.
Claiming "Exempt" on the W-4, usually means that the employer will withhold no Income taxes (though if the child is expected to earn a lot..that "exempt" claim might not be allowed). As long as she only earns $6150 or less during the year (from that job and all other income she might have for the year), you should be OK.
But if she earns considerably more than that, and you used the "Exempt" designation, then when you file her income tax return at tax time, then you would need to send $$ in to the IRS. That amount would be fairly minor if she earned less than $10,000 during the year, but you would have to send the IRS some $$ at tax time
Putting a "zero" in for allowances on the W-4, would mean that the employer would withhold income taxes at the highest level. And then, for that situation, even if she earns less than 6150, you would need to file a tax return for her...but only if you wanted to get the tax withholding refunded.
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*
