procedure FillLevel(Parent: PVirtualNode);
var
  group: PGroup;
  n: PVirtualNode;
begin
  if Parent <> nil then // Это не корневая группа
      begin
        group := ATree.GetNodeData(Parent);
        ATree.DeleteChildren(Parent);
      end
  else
        group.id := 0;

  Q.SQL.Text := 'SELECT * FROM... WHERE up_id = ' + group.id;
  ....
  while not Q.EOF do
      begin
          n := ATree.AddChild(Parent);
          group := ATree.GetNodeData(n);
          group.id := Q.FieldByName('id').AsInteger;
          group.up_id := Q.FieldByName('up_id').AsInteger;
          group.name := Q.FieldByName('name').AsString;
          Q.Next;
      end;
end;

procedure GroupsTreeInitNode(Sender: TBaseVirtualTree;
  ParentNode, Node: PVirtualNode; var InitialStates: TVirtualNodeInitStates);
begin
  FillLevel(Sender, Node);
end;

  Include(Node.States, vsHasChildren);
  Node.ChildCount := q.Q.FieldByName('c').AsInteger;