Finding Control In Collection Tree?

i am adding dynamic control (textbox) and i set the property to visible = false but i dont find the control in the tree collection, i want to hide it from user seeing it and read the value.

protected void gv_RowCreated(object sender, GridViewRowEventArgs e)
{
Students org = (namespace.Students )(e.Row.DataItem);
foreach (Registration reg in org.Registrations)
{
int _count = org.Registrations.Count;
for (int rowId = 0; rowId < _count; rowId++)
{
TextBox txtBox = new TextBox();
txtBox.ID = "_registration" + e.Row.RowIndex + "_" + rowId;
txtBox.Text = reg.Name;
txtBox.Visible = true;
e.Row.Cells[7].Controls.Add(txtBox);
}
}
}