COLUMBIA, S.C. — For most of the campaign, Nikki Haley’s children have stayed off the stump. Outside of the campaign kickoff where Haley’s family joined her at the end of her speech, the public role of her children in the campaign has been limited. At most Nalin Haley, a senior in college, writes weekly fundraising appeals to encourage supporters to contribute $5 at a time. Rena Haley Jackson, the presidential candidate’s daughter who is an Upstate pediatric nurse, has knocked on doors for the campaign and was often in the front row of debates to support her mother. The two children have sat f…