コンテナーコントロール以外の他のコントロール内にもTextBoxを配置できることを忘れないでください。TextBoxをPictureBoxに追加することもできます。
だからあなたもチェックする必要があります
someControl.HasChildren = True
任意の再帰関数で。
これは、このコードをテストするためにレイアウトから得た結果です。
TextBox13   Parent = Panel5
TextBox12   Parent = Panel5
TextBox9   Parent = Panel2
TextBox8   Parent = Panel2
TextBox16   Parent = Panel6
TextBox15   Parent = Panel6
TextBox14   Parent = Panel6
TextBox10   Parent = Panel3
TextBox11   Parent = Panel4
TextBox7   Parent = Panel1
TextBox6   Parent = Panel1
TextBox5   Parent = Panel1
TextBox4   Parent = Form1
TextBox3   Parent = Form1
TextBox2   Parent = Form1
TextBox1   Parent = Form1
tbTest   Parent = myPicBox
フォーム上の1つのボタンと1つのRichTextBoxでこれを試してください。
Option Strict On
Option Explicit On
Option Infer Off
Public Class Form1
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim pb As New PictureBox
        pb.Name = "myPicBox"
        pb.BackColor = Color.Goldenrod
        pb.Size = New Size(100, 100)
        pb.Location = New Point(0, 0)
        Dim tb As New TextBox
        tb.Name = "tbTest"
        pb.Controls.Add(tb)
        Me.Controls.Add(pb)
        Dim textBoxList As New List(Of Control)
        textBoxList = GetAllControls(Of TextBox)(Me)
        Dim sb As New System.Text.StringBuilder
        For index As Integer = 0 To textBoxList.Count - 1
            sb.Append(textBoxList.Item(index).Name & "   Parent = " & textBoxList.Item(index).Parent.Name & System.Environment.NewLine)
        Next
        RichTextBox1.Text = sb.ToString
    End Sub
    Private Function GetAllControls(Of T)(ByVal searchWithin As Control) As List(Of Control)
        Dim returnList As New List(Of Control)
        If searchWithin.HasChildren = True Then
            For Each ctrl As Control In searchWithin.Controls
                If TypeOf ctrl Is T Then
                    returnList.Add(ctrl)
                End If
                returnList.AddRange(GetAllControls(Of T)(ctrl))
            Next
        ElseIf searchWithin.HasChildren = False Then
            For Each ctrl As Control In searchWithin.Controls
                If TypeOf ctrl Is T Then
                    returnList.Add(ctrl)
                End If
                returnList.AddRange(GetAllControls(Of T)(ctrl))
            Next
        End If
        Return returnList
    End Function
End Class