She can be self-employed.
Generally, for 2016 and 2017, an employer needs to pay Social Security and Medicare taxes for “cash wages” of $2,000 or more paid to any one employee. Cash wages refer to checks, money orders and the like. They don’t include “the value of food, lodging, clothing, transit passes and other noncash items you give your household employee.” But cash given to an employee in place of those items counts as cash wage.
Don’t count wages paid to your spouse or your child under 21, the IRS says. Wages paid to your parent typically don’t apply, either, although there are exceptions.
Don’t count wages paid to an employee under 18 at any time during the year, unless providing household services is “the employee’s principal occupation,” the IRS says.