Dim frm As New FormChild
 
Private Sub Form_Load
     frm.show()
End Sub
 
Private Sub Button1_Click   'Liegt auf FormParent
    Dim i As Integer = 1
    Do While i <= 3
       ' frm.TextBox.AppendText(Controls("frm.Label" & i).Text & vbCrLf)
        frm.TextBox.Text = frm.Label.Text & i.ToString & vbCrLf
        i +=  1
    Loop
End Sub