If you are showing your child form as a Modal dialog (using ShowDialog) then you can set the StartPosition property to CenterParent and it will (initially) be correct.
If not, then you will have to do it manually:
Set the StartPosition to Manual
When you first show it, set it's Location relative to the current form
frmChild f = new frmChild();
f.Top = ((Height - f.Height) / 2) + Top;
f.Left = ((Width - f.Width) / 2) + Left;
f.Show();
You will also have to handle the parent form Resize event and do the same there.
[edit]Why did I say "progress bar" instead of "child form"? Need coffee...[/edit]