Children and FICA withholding rules
If a child under age 18 is paid a wage from a sole proprietorship in which one parent is an owner or a partnership in which each owner is a parent, no FICA withholding is required. However, if the wage is paid from a corporation, the IRS disallows this benefit.
For more information, visit https://www.irs.gov/businesses/small-businesses-self-employed/family-help.