I think that You can do it in few steps:
1. Store base.ActiveMdiChild of Your MDI main form.
2. Show new MDI child
3. Call stored MDI child form's method Activate()
If this sample show flickering of "unwanted" form, you
can apply simple trick.
1.1. Set new MDI child WindowState to minimized
4. Set new MDI child WindowState to Normal (or maximized)