Seri Kertzner left her corporate job to become a party planner.Courtesy of Seri KertznerSeri Kertzner left J.Crew after her first son was born.She founded Little Miss Party and later expanded to three brands.She says entrepreneurship gave her more flexibility to prioritize her children.This as-told-to essay is based on a conversation with Seri Kertzner, 47, who lives in Florida. She spent a decade working in the corporate fashion industry in New…