@AmeliesUncle is correct. Even though an S corp is a pass through entity, for income taxes purposes, it is still a corporation for employee payroll purposes. The underage children do not qualify FICA exemption. I've revised my answer above.
Here's a possible workaround: https://wealthfactory.com/articles/how-to-avoid-payroll-tax-for-your-children-if-your-business-is-an...
