Very grey area. You can't pay them to run errands, mow the lawn, etc. I'd suggest that you would have to have self employment income (i.e. your own business) for you to justify the deduction. And then the work itself needs to be applicable to the business.
I've commented here and elsewhere that I jumped on this when my daughter at age 12 started to have income from babysitting. I told her that in exchange for her taking the time to keep a notebook, listing the family paying her, the date, and amount paid, I'd make a deposit to a Roth IRA for her.
I've approaches taxes each year in a way that would be audit-compliant, i.e. a paper trail that covers any and all deductions, donations, etc. In the real world, the IRS isn't likely to audit someone for that Roth deposit, as there's little for them to recover.