Hi,

you can get the child items through

        ms.GetPagedChildren(item.Id, 0, 100, out long total);

this would get the first 100 items (nothing but performance stopping you upping the pageSize variable)

a more comprehensive way to get everything

const int pageSize = 500;
var page = 0;
var total = long.MaxValue;
while(page * pageSize < total)
{
  var children = ms.GetPagedChildren(item.Id, page++, pageSize, out total);

  // do stuff wit the child items
}

Also GetPagedDescendants will get all items down the tree.

Update: Just getting folders is a little more complex, but you can see how Umbraco does it in the source for the media controller https://github.com/umbraco/Umbraco-CMS/blob/v8/contrib/src/Umbraco.Web/Editors/MediaController.cs#L196