You'll need to keep references to all the child forms in the main form (when you create them, maybe) and then you will have access to the forms when you want it.
<br />
Form newForm = new Form();<br />
<br />
... Open the form, etc...<br />
<br />
... Access the newForm control list ...<br />
foreach (Control c in newForm.Controls)<br />
{<br />
listboxB.Items.Add(c.Name);<br />
}<br />
<br />
<br />
<br />