<SimpleTreeView defaultExpandedItems={['3']}>
  <CustomTreeItem itemId="1" label="Amelia Hart">
    <CustomTreeItem itemId="2" label="Jane Fisher" />
  </CustomTreeItem>
  <CustomTreeItem itemId="3" label="Bailey Monroe">
    <CustomTreeItem itemId="4" label="Freddie Reed" />
    <CustomTreeItem itemId="5" label="Georgia Johnson">
      <CustomTreeItem itemId="6" label="Samantha Malone" />
    </CustomTreeItem>
  </CustomTreeItem>
</SimpleTreeView>