You can remove all the children of a group or a panel in a scriptUI panel using this function. Just pass that object to the function call

removeChildrenFromScriptUIItem.jsx

function RemoveAllChildrenFromPanel (panelObj) {
  var panelObj = panelObj || undefined;

  if (!panelObj || !panelObj.children || !panelObj.children.length) {
    // no panel, or no children found
    return;
  }

  for (var i = panelObj.children.length - 1; i >= 0; i--) {
    var crntChildren = panelObj.children[i];
    if (!crntChildren) {
      continue;
    }
    try {
      panelObj.remove(crntChildren);
    } catch (e) {
      // something went wrong with removing this children, you can print or alert an error here
      continue;
    }
  }
};