Since the F1 shortcut has already been assigned to a menu item in the main MDI form, you cannot use it anymore in its MDI children form.
You can however do some simple settings change to make it work.
I assume you want the F1 do different action while the main MDI form is focused and another different action when the MDI children is focused. If that is so then you can do the following.
First at main MDI form, when opening the children form, remove the F1 shortcut key
Private Sub TestToolStripMenuItem_Click(ByVal sender As Object, ByVal e As EventArgs) Handles TestToolStripMenuItem.Click
TestToolStripMenuItem.ShortcutKeys = Keys.None
Dim f As New Form2
f.MdiParent = Me
f.Show()
End Sub
Then when that children form closes, just assign the shortcut key back
Private Sub Form2_FormClosing(ByVal sender As Object, ByVal e As FormClosingEventArgs) Handles Me.FormClosing
TryCast(Me.MdiParent, MDIForm).TestToolStripMenuItem.ShortcutKeys = Keys.F1
End Sub
Private Sub Form2_KeyDown(ByVal sender As Object, ByVal e As KeyEventArgs) Handles Me.KeyDown
If e.KeyCode = Keys.F1 Then
End If
End Sub