I don't know if this is convenient for you, but you could add a Panel/FlowLayoutPanel
to your Mdi window in the desired child forms space and add the forms to the panel like this :
Form frm = New Form();
frm.TopLevel = False;
frm.Show();
FlowLayoutPanel1.Controls.Add(frm);
Set the FlowLayoutPanel.BackColor
to Transparent
so it looks like an mdi container.